Diffractive optical beam shaping element

ABSTRACT

A diffractive optical beam shaping element for imposing a phase distribution on a laser beam that is intended for laser processing of a material includes a phase mask that is shaped as an area and is configured for imposing a plurality of beam shaping phase distributions on the laser beam incident on to the phase mask. A virtual optical image is attributed to at least one of the plurality of beam shaping phase distributions, wherein the virtual image can be imaged into an elongated focus zone for creating a modification in the material to be processed. Multiple such elongated focus zones can spatially add up and interfere with each other, to modify an intensity distribution in the material and, for example, generate an asymmetric modification zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority under 35U.S.C. §120 to PCT Application No. PCT/EP2015/076708 filed on Nov. 16,2015, which claims priority to German Application No. 10 2014 116 958.1,filed on Nov. 19, 2014. The entire contents of these priorityapplications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to diffractive optical elements that areused in optical systems for beam shaping a laser beam and in particularfor beam shaping a laser beam for processing materials that areessentially transparent for the laser beam. Moreover, the inventionrelates to a method for laser material processing.

BACKGROUND

There are many possibilities for using absorption of light forprocessing a work-piece, in particular by introducing localizedmodifications into the work-piece. The so-called volume absorption,i.e., an absorption that is not limited to the surface, opens thepossibility to process brittle-hard materials that are essentiallytransparent for the laser beam. Generally, volume absorption benefitsfrom a kind of nonlinear absorption, at which an interaction with thematerial takes place only at a material dependent (threshold) intensity.

SUMMARY

Herein, a nonlinear absorption is understood as an intensity dependentabsorption of light, that is not primarily based on the directabsorption of the light. Instead it is based on an increase of theabsorption during interaction with the incident light, often atemporally limited laser pulse. Thereby, electrons can absorb that muchenergy by inverse bremsstrahlung that further electrons are set free byimpacts, so that the rate of generating electrons overcomes that rate ofrecombination. Under specific conditions, those initial electrons, whichare required for the avalanche-like absorption, may already be presentfrom the start or may be generated by an existing rest-absorption bylinear absorption. For example, for ns-laser pulses, an initialionization may result in an increase in temperature that causes anincrease of the number of free electrons and therefore of the followingabsorption. Under other conditions, such initial electrons may begenerated by multi-photon ionization or tunnel ionization as examples ofwell-known nonlinear absorption mechanisms. For ultrashort laser pulseswith, for example, sub-ns-pulse durations such an avalanche-likegeneration of electrons can be utilized.

A volume absorption may be used for materials, which are essentiallytransparent for the laser beam (herein in short referred to astransparent materials), for forming a modification of the material in anelongated focus zone. Such modifications may allow separating, drilling,or structuring of the material. For separating, for example, rows ofmodifications may be generated that cause a breaking within or along themodifications. Moreover, it is known to generate modifications forseparating, drilling, and structuring that allow a selective etching ofthe modified areas (SLE: selective laser etching).

The generation of an elongated focus zone can be affected with the helpof apodized Bessel beams (herein also referred to as quasi-Bessel beam).Such beam profiles may be formed, for example, with an axicon or aspatial light modulator (SLM: spatial light modulator) and an incidentlight beam having a Gaussian beam profile. A subsequent imaging into atransparent work-piece results in the intensities required for volumeabsorption. Quasi-Bessel beams—like Bessel beams—usually have aring-shaped intensity distribution in the far field of the beam profileexisting within the work-piece. Calculating phase distributions for beamshaping quasi-Bessel beams, e.g., with an SLM is disclosed in Leach etal., “Generation of achromatic Bessel beams using a compensated spatiallight modulator,” Opt. Express 14, 5581-5587 (2006), the entire contentsof which are incorporated by reference.

Moreover, systems are known for forming a line of intensityenhancements, e.g., with the help of multifocal lenses. Thereby, a phasemodification of the laser beam to be focused is performed in the farfield, i.e., during focusing, whereby the phase modification results inthe formation of longitudinally displaced focus zones.

An aspect of the present disclosure has the objective to provide adiffractive optical beam shaping element that enables beam shaping for atailored volume absorption. In particular, the objective is, for laserprocessing applications, to provide in beam propagation directionelongated, slender beam profiles with a high aspect ratio for processingtransparent materials.

At least one of the objectives is solved by a diffractive optical beamshaping element of claim 1, an optical system of claim 10, a laserprocessing machine of claim 12, and a method for material processing alaser beam of claim 22. Further developments are given in the dependentclaims.

In an aspect, a diffractive optical beam shaping element for imposing aphase distribution on a laser beam, which is intended for laserprocessing of a material, includes an areally configured phase mask thatis configured for imposing a plurality of beam shaping phasedistributions on the laser beam that is incident on to the phase mask. Avirtual optical image is attributed to at least one of the plurality ofbeam shaping phase distributions, wherein the virtual image can beimaged into an elongated focus zone for creating a modification in thematerial to be processed.

In another aspect, an optical system for beam shaping of a laser beamfor processing an in particular transparent material by modifying thematerial in a common focus zone being elongated in propagation directionincludes such a diffractive optical beam shaping element and a nearfield optics located downstream of the diffractive optical beam shapingelement at a beam shaping distance and configured to focus the laserbeam into the focus zone. Thereby, at least one imposed phasedistribution of the plurality of beam shaping phase distributions issuch that a virtual optical image of an elongated focus zone isattributed to the laser beam, the optical image being located before thediffractive optical beam shaping element. The beam shaping distancecorresponds to a propagation length of the laser beam within which theplurality of beam shaping phase distributions transform the transverseinput intensity profile into a transverse output intensity profile inthe region of the near field optics. In particular, the transverseoutput intensity profile has, in comparison with the input intensityprofile, at least one local maximum positioned outside of the beam axis.

In a further aspect, a method for material processing a material, whichis in particular to a large extent transparent for the laser beam, bymodifying the material with a laser beam includes the following steps:imposing a plurality of beam shaping phase distributions onto atransverse input intensity profile of the laser beam, wherein at leastone of the imposed phase distributions is such that a virtual opticalimage of an elongated focus zone is attributed to the laser beam;propagating the laser beam over a beam shaping distance, after which theplurality of imposed beam shaping phase distributions has transferredthe transverse input intensity profile into a transverse outputintensity profile, so that the transverse output intensity profile, incomparison to the input intensity profile, includes in particular atleast one local maximum located outside of the beam axis; focusing thelaser beam into the focus zone for forming a near field, which is basedon the output intensity profile, while superposing, adding, and/orinterfering the elongated focus zone attributed to the virtual opticalimage with at least one further focus zone, which is based on at leastone further phase distribution of the plurality of beam shaping phasedistributions.

Herein, concepts are disclosed that allow to at least partly improveaspects of the prior art. In particular, additional features and theirfunctionalisms result from the following description of embodiments onthe basis of the drawings. The drawings show:

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of an optical system for beam shapingof a laser beam;

FIG. 2 is a schematic illustration of a laser processing device with anoptical system according to FIG. 1 for material processing;

FIG. 3 is a schematic illustration of an optical system for explainingthe optical functioning;

FIG. 4 is an example of a longitudinal intensity distribution in anelongated focus zone after imaging a virtual optical image;

FIG. 5 is a ZR-section of the longitudinal intensity distribution shownin FIG. 4;

FIG. 6 is an exemplary experimental study on the modification of atransparent material in an elongated focus zone according to FIGS. 4 and5;

FIG. 7 is a schematic illustration for explaining the generation andimaging of a real intensity enhancement,

FIG. 8 is an example of a longitudinal intensity distribution in anelongated focus zone after imaging a real intensity enhancementaccording to FIG. 7;

FIGS. 9, 10, 11A and 11B are schematic illustrations of examples foroptical systems based on transmitting or reflective axicons;

FIG. 12 is a schematic illustration of an example of an optical systembased on a spatial light modulator;

FIG. 13 is a schematic illustration of an example of an optical systembased on a transmitting diffractive optical element;

FIG. 14 is a schematic illustration of an example of a phasedistribution in a diffractive optical element in an optical systemaccording to FIG. 13;

FIG. 15 is an exemplary intensity cross-section of an output intensityprofile in an optical system according to FIG. 13;

FIG. 16 is an XY-view of the output intensity profile of the intensitycross-section shown in FIG. 15;

FIG. 17 is a schematic illustration of an example of an optical systemwith filtering non-phase-modulated beam portions;

FIG. 18 is a schematic illustration of an example of an optical systembased on a diffractive optical element with a linear phase contributionfor separating a phase-modulated beam portion;

FIG. 19 is a schematic illustration of an example of an optical systemwith a scan device;

FIG. 20 is a schematic illustration for explaining the imaging system ofan optical system;

FIG. 21 is a schematic illustration for explaining an optical system forthe incidence of a converging laser beam;

FIG. 22 is a schematic illustration for explaining an optical systemwith adaptation of the divergence;

FIG. 23 is an exemplary cross-section of the intensity of an outputintensity profile in an optical system for generation of a flat-topintensity profile;

FIG. 24 is an XY-view of the output intensity profile of the intensitycross-section shown in FIG. 23;

FIG. 25 is an example of a longitudinal intensity distribution thatresults from the output intensity profile of FIGS. 23 and 24;

FIG. 26 is an exemplary experimental study on the modification of atransparent material in an elongated focus zone according to FIG. 25;

FIG. 27 is an example of a longitudinal intensity distribution whenusing a multifocal near field optics;

FIG. 28 is a schematic illustration of an example of a phasedistribution for generating an inverse Airy beam (e.g., inversequasi-Airy like beam) shape with a diffractive optical element for usein an optical system according to FIG. 13;

FIG. 29 is an exemplary intensity cross-section of an output intensityprofile for generating the inverse Airy beam shape according to FIG. 28;

FIG. 30 is an XY-view of the output intensity profile of the intensitycross-section shown in FIG. 29;

FIG. 31 is an example of a longitudinal intensity distribution in anelongated focus zone for the inverse Airy beam shape generated with thephase distribution according to FIG. 28;

FIG. 32 is a schematic illustration for explaining the imaging of avirtual image in combination with the imaging of a real intensityenhancement;

FIGS. 33A, 33B, 33C and 33 are D beam profiles for an inversequasi-Bessel beam at the propagation from the beam shaping element tothe near field optics;

FIG. 34is an amplitude distribution for a section along the beam axis Zfor illustration of the positions of the beam profiles of FIGS. 33A to33D; and

FIGS. 35A and 35B are radially segmented phase distributions;

FIG. 36 is an amplitude distribution for a cut along the beam axis Zwhen propagating from the beam shaping element to the near field opticsfor a phase imposing of FIG. 35B;

FIGS. 37A, 37B and 37C are beam profiles at z=10 mm, z=50 mm, z=200 mmfor a phase imposing of FIG. 35B;

FIG. 38A, 38B and 38C are transverse intensity distributions at z=10 mm,z=50 mm, z=200 mm for a phase imposing of FIG. 35B;

FIG. 39 is a longitudinal intensity distribution in an elongated focuszone for a phase imposing of FIG. 35B;

FIG. 40 is a ZR-cut of the longitudinal intensity distribution shown inFIG. 39;

FIG. 41 is an exemplary experimental study on the modification of atransparent material in an elongated focus zone according to FIGS. 39and 40;

FIG. 42 is an azimuthal segmented phase distribution;

FIG. 43 is an exemplary intensity cross-section for a phase imposing ofFIG. 42;

FIG. 44 is an XY-view of the output intensity profile of the intensitycross-section shown in FIG. 43;

FIG. 45 is a ZX-cut of an elongated focus zone for a phase imposing ofFIG. 42;

FIG. 46 is a ZY-cut of an elongated focus zone for a phase imposing ofFIG. 42;

FIG. 47 is a phase distribution for generating two transverse displacedinverse quasi-Bessel beam profiles;

FIG. 48 is an amplitude distribution for a cut along the beam axis Zwhen propagating from the beam shaping element to the near field opticsfor a phase imposing of FIG. 47;

FIG. 49 is an XY-view of the output intensity profile for a phaseimposing of FIG. 47;

FIG. 50 is a ZX-cut of an elongated focus zone for a phase imposing ofFIG. 47; FIG. 51A, 51B and 51C are beam profiles at z=10 mm, z=100 mm,z=150 mm for a phase imposing of FIG. 47; and

FIG. 52A, 52B and 52C are transverse intensity distributions in Xdirection at z=10 mm, z=100 mm, z=150 mm for a phase imposing of FIG.47.

DETAILED DESCRIPTION

Aspects described herein are based partly on the realization that, dueto the high intensities needed for laser processing, intensities may bepresent already during the preparation of the laser beam that result indamage of optical elements. In view thereof, it was further realizedthat the generation of an elongated focus zone within the work-piece maybe based on the imaging of a virtual beam profile. By this concept ofimaging a virtual beam profile, regions with intensity peaks can bereduced or even avoided in the optical system. It was further realizedthat a phase distribution attributed to the virtual beam profile may beimposed onto the laser beam that causes the desired change of theintensity distribution in the far field. In particular, it was realizedthat by a far field distribution, which originates from such a virtualbeam profile, for example, inverse-Bessel beam-like or inversequasi-Airy beam-like intensity distributions, specifically designedintensity distributions, and in particular superpositions of the same inthe focus zone can be created. For such intensity distributions, alateral energy entry into the focus zone can take place, which inparticular enables the processing of transparent materials. It wasfurther realized that, in comparison to systems for imaging a realintensity enhancement, the concept of the imaging of a virtual beamprofile may lead to shorter configurations of such optical systems.

An elongated focus zone relates herein to a three-dimensional intensitydistribution defined by the optical system that determines the spatialextent of the interaction and thereby the modification within the to beprocessed material. The elongated focus zone determines thereby anelongated region in which a fluence/intensity is present within the tobe processed material, which is beyond the threshold fluence/intensitybeing relevant for the processing/modification. Usually, one refers toelongated focus zones if the three-dimensional intensity distributionwith respect to a target threshold intensity is characterized by anaspect ratio (extent in propagation direction in relation to the lateralextent) of at least 10:1, for example 20:1 and more, or 30:1 and more.Such an elongated focus zone can result in a modification of thematerial with a similar aspect ratio. In some embodiments, focus zonescan be formed that are, for example, also in propagation directionparallel with respect to each other, wherein each of the focus zones hasa respective aspect ratio. In general, for such aspect ratios, a maximalchange of the lateral extent of the (effective) intensity distributionover the focus zone can be in the range of 50% and less, for example 20%and less, for example in the range of 10% and less.

Thereby, the energy within an elongated focus zone can be laterallysupplied essentially over the complete length of the createdmodification. As a consequence, a modification of the material in theinitial region of the modification does not have or hardly has anyshielding effects on the part of the laser beam that causes amodification of the material downstream of the beam, i.e., for example,in the end region of the modification zone. In that sense, a Gaussianbeam cannot generate a comparable elongated focus, because the energysupply is performed essentially longitudinally and not laterally.

The transparency of a material, which is essentially transparent for alaser beam, relates herein to the linear absorption. For light below thethreshold fluence/intensity, material, which is essentially transparentfor a laser beam, may absorb, for example, along a length up to the backend of the modification, e.g., less than 20% or even less than 10% ofthe incident light.

Aspects described herein further are partly based on the realizationthat by a desired beam shaping, for example, with a diffractive opticalelement (DOE), the density of free electrons, which is created in thematerial by nonlinear absorption, may be tailored. Along the therebycreated modifications, a crack formation may be specifically guided,which then results in the separation of the work-piece.

Aspects described herein further are based partly on the realizationthat, for a DOE, multiple phase distributions can be provided in thephase distribution of a phase mask, for example, in respective segments.Thereby, in particular the advantages of the concept of a virtualoptical image, for example, an inverse quasi-Bessel beam (e.g., inversequasi-Bessel like beam) shape, can be used at the superposition of theimaging of multiple such virtual images (in longitudinal or lateraldirection), wherein also the interaction (e.g., interference) andspatial constellation of multiple imaging may have effects onto theformation of the common focus zone. In addition, it was recognized thatin this manner asymmetric “common” focus zones can be created. Forexample, for material processing, asymmetric “common” focus zones createa preference for a specific movement direction or a specific separationdirection. Moreover, it was recognized that, during the laserprocessing, such preferred directions may be adopted to desiredprocessing trajectories by orienting/turning the DOE within an opticalsystem. For digital phase masks (SLMs etc.), a direct controlling of thephase distribution may further be performed to adapt the preferreddirection.

Aspects described herein further are based in part on the realizationthat, by the use of a DOE, additional phase distributions may be imposedonto the beam, which, for example, may simplify the setup of theunderlying optical systems and/or the isolation of a usable beamportion.

In other words, disadvantages of the prior art may in some embodimentsat least partly be overcome by an optic concept, in which the beamprofile, which is positioned in the region of the work-piece and whichis elongated in propagation direction, is affected by an imaging of acreated virtual beam profile. In some embodiments, the optic conceptfurther allows a filtering possibility for undesired beam portions, forexample, in a region of the Fourier-plane of the beam profile and aseparation of the beam shaping from the focusing.

The systems and methods resulting from these realizations can inter aliaenable separating of transparent, brittle-hard materials with highvelocity and with good quality of the cutting edge. Moreover, suchsystems and methods may further enable separating without a taper angleas it is created in ablating methods. In particular, when separatingbased on non-ablating modifications, there may be no or only a smallremoval, with the consequence that the material has only a few particleson the surface after the processing.

In the following, the underlying optical concept will be generallyexplained with reference to FIGS. 1 to 8. Then, exemplary embodiments ofoptical systems will be explained, which, on the one side, implement theoptical system by conventional optics such as lenses and mirrors (seeFIGS. 9 to 11B) and, on the other side, by diffractive optical elements(see FIGS. 12 to 16). In connection with FIGS. 17 to 22, thecombinability of the optical system with components and aspects forfiltering and scanning as well as general aspects of the beamdevelopment within the optical system are explained. Finally, inconnection with FIGS. 23 to 32, exemplary embodiments of the elongatedfocus zones for material processing are illustrated, which in particularcan be realized with diffractive optical elements. In FIGS. 33A to 33Dand 34, beam profiles and a longitudinal amplitude distribution areexplained for an inverse quasi-Bessel beam at the propagation from thebeam shaping element to the near field optics in the optical system.

In the remaining figures, various concepts are proposed that relate tothe interference of inverse quasi-Bessel beams in e.g., longitudinaldisplaced focus zones (FIGS. 35A to 41) and to the formation oftransverse asymmetries due to azimuthal segmentation (FIGS. 42 to 46)and transverse displaced phase distributions (FIGS. 47 to 52C).

FIG. 1 shows a schematic illustration of an optical system 1 for beamshaping a laser beam 3 with the aim to create a focus zone 7, which iselongated in a propagation direction 5, within a to be processedmaterial 9. Generally, laser beam 3 is determined by beam parameterssuch as wavelength, spectral width, temporal pulse shape, formation ofpulse groups, beam diameter, transverse input intensity profile,transverse input phase profile, input divergence, and/or polarization.According to FIG. 1, laser beam 3 is supplied to optical system 1 forbeam shaping, i.e., for transforming one or more of the beam parameters.Usually, for laser material processing, laser beam 3 will be acollimated Gaussian beam with a transverse Gaussian intensity profile,which is generated by a laser beam source 11, for example an ultrashortpulse high-intensity laser system. The transformation can be performed,for example, into an inverse Bessel or inverse Airy beam shape.

In the laser processing machine 21 shown in FIG. 2, optical system 1may, for example, be used for material processing. Laser processingmachine 21 includes a support system 23 and a work-piece positioningunit 25. Support system 23 spans over work-piece positioning unity 25and carries laser system 11, which is integrated in FIG. 2, for example,in an upper crossbeam 23A of support system 23. In addition, opticalsystem 1 is mounted at crossbeam 23A to be displaceable in X direction,so that both components are arranged close to each other. In alternativeembodiments, laser system 11 may be provided, for example, as a separateexternal unit, laser beam 3 of which is guided to optical system 1 byoptical fibers or as a free propagating beam.

Work-piece positioning unit 25 carries a work-piece that extends in theX-Y-plane. The work-piece is the to be processed material 9, forexample, a glass plate or a plate in ceramic or crystalline embodimentsuch as sapphire or silicon, that is essentially transparent for thelaser wavelength used. Work-piece positioning unit 25 allows displacingthe work-piece in Y direction relative to support system 23, so that, incombination with the displaceability of optical system 1, a processingarea is provided, which extends within the X-Y-plane.

According to FIG. 2, in addition, a relocatability is provided of e.g.,optical system 1 or crossbeam 23A in Z direction, such that the distanceto the work-piece can be set. For a cut running in Z direction, thelaser beam is usually also directed in the Z direction (i.e., normal)onto the work-piece. However, additional processing axes may be providedas exemplarily illustrated in FIG. 2 by the boom arrangement 27 and theadditional rotational axes 29. Accordingly, boom arrangement 27 isoptional in the embodiment of

FIG. 2. In addition, redundant add-on axes may be provided for higherdynamics, as, for example, not the work-piece or the optical system, butmore compact and respectively designed components are accelerated.

Laser processing machine 21 further includes a control unit notexplicitly shown in FIG. 1, which is, for example, integrated withinsupport system 23 and which in particular includes an interface forinputting operation parameters by a user. In general, the control unitincludes elements for controlling electrical, mechanical, or opticalcomponents of laser processing machine 21, for example, by controllingrespective operation parameters such as pump laser power, cooling power,direction and velocity of the laser machine and/or the work-piecepositioning unit, electrical parameters for setting an optical element(for example, of an SLM) and the spatial orientation of an opticalelement (for example, for rotation of the same).

Additional arrangements for laser processing machines with variousdegrees of freedom are disclosed, for example, in EP 1 688 807 A1, theentire contents of which are incorporated by reference. In general, forsmaller work-pieces often only the work-piece is moved, and for largerwork-pieces only the laser beam or—as in FIG. 2—the work-piece and thelaser beam are moved. Moreover, two or more optical systems and, thus,focus zones may be supplied by a single laser system 11.

The modifications within the material, which are generated by the laserprocessing machine, may be used, for example, for drilling, separatingby induced tensions, welding, creating a modification of the refractionbehavior, or for selective laser etching. Accordingly, it is importantto control the geometry as well as the type of modification in asuitable manner. Besides parameters such as laser wavelength, temporalpulse shape, number of pulses, energy and temporal distance of thepulses within a pulse group creating an individual modification, as wellas pulse energy or pulse group energy, the beam shape plays a decisiverole.

In particular, an elongated volume modification allows processing of a,in beam propagation direction long extending, volume region within asingle processing step. In particular, at one position in feeddirection, the processing can take place over a large extent in only asingle modification processing step. By the use of the optical systemsdescribed herein, beam shapes, and methods, one can achieve, on the oneside, better work results (in comparison to single modifications thatare positioned next to each other at one position in feed direction insucceeding modification processing steps) and, on the other side, onecan reduce the processing time and the requirements for the systemtechnology. Then, for single modifications, multiple working steps areneeded that increase the time needed and that require a more involvedensuring of relative positions of the single modifications.

In addition, an elongated focus zone can be helpful when processinguneven materials, because essentially identical laser processingconditions are given along the elongated focus zone such that, in thoseembodiments, a respective readjusting in propagation direction may notbe necessary or only be necessary starting at a larger deviation of theposition of the to be processed material than the lengths of theelongated focus area (in consideration of the requiredprocessing/intrusion depth).

In general, it applies to the processing of transparent materials byelongated volume absorption that, as soon as absorption takes place,that absorption itself or the resulting changes in the materialproperties can influence the propagation of the laser beam. Therefore,it is advantageous, if beam portions, which should cause a modificationdeeper within the work-piece, i.e., in beam propagation directiondownward, essentially propagate not through regions of considerableabsorption.

In other words, it is favorable to lead those beam portions, whichcontribute to the modification further downward, under an angle to theinteraction zone. An example for this is the quasi-Bessel beam, forwhich a ring-shaped far-field distribution is given, the ring width ofwhich is typically small in comparison to the radius. Thereby, the beamportions of the interaction zone are led in essentially with that anglein rotational symmetry. The same applies for the inverse quasi-Besselbeam described herein or for modifications or extensions of the samesuch as the homogenized or modulated inverse quasi-Bessel beam. Anotherexample is the inverse accelerated “quasi-Airy beam-like” beam, forwhich the beam portions are led into the modification under an offsetangle, where this is done clearly tangential and—not as for the purequasi-Bessel beam rotationally symmetric—to the curved modificationzone, e.g., as for a curved inverse quasi-Bessel beam.

Moreover, it is desired to considerably pass the threshold for thenonlinear absorption only within the desired volume region and to choosethe geometry of that volume area such that it is suitable for thedesired application, but that also the propagation to further downstreampositioned volume regions is not significantly disturbed. For example,it may be advantageous to keep secondary maxima of an apodized Besselbeam profile below a threshold intensity needed for nonlinearabsorption.

In view of modifications being subsequent in the feed direction, thegeometry of the modified volume may further be selected such that, for arow of multiple modifications in the feed direction, an earlier inducedmodification has only an insignificant influence on the formation of thefollowing modifications.

As already mentioned, for fast processing, the generation of a singlemodification can be performed with only a single laser pulse/a singlelaser pulse group, so that a position on a work-piece is approached onlyonce in this case.

Ultrashort pulse lasers can make intensities (power densities) availablethat allow causing a sufficiently strong material modification inrespective long interaction zones. The geometric extent of themodification is thereby set with the help of beam shaping such that along extending, high density of free electrons is created by nonlinearabsorption in the material. The supply of energy in deeper regions isperformed laterally, so that the shielding effect by an upstreaminteraction of the plasma can be avoided in comparison to a Gaussianfocusing. For example, an electron density, which extends smoothly inlongitudinal direction, or an electron density, which is modulatedspatially with a high frequency, can be generated.

At the respective intensities, within regions with a sufficiently highdensity of free electrons, an explosive expansion of the material may becaused, whereby the thereby resulting shock-wave can create nanoscopicholes (nano-voids). Additional examples for modifications (modificationzones) are changes in the refractive index, compressed and/or tensilestress induced regions, micro-crystallites, and local changes instoichiometry.

As explained at the beginning, by the accumulation of such modificationzones in feed direction, a course of a crack can be set. Duringprocessing, the work-piece is accordingly separated along a respectivemodified contour. The crack formation can then occur directly thereafteror can be induced by another process. For example, for the separation ofnon-pre-strained materials ultrasound ramps, or temperature ramps may beused in order to cause a later separation along the modified contour. Asingle modification usually does not lead to crack formation.

With the help of a tailored beam shape, various tension distributionswithin the material and between the modified regions can be created inorder to adapt the separation process to a given material. In theprocess, strong spatial and temporal gradients can favor the formationof a micro- or nano-explosion.

The modification geometry is thereby primarily determined by the beamshaping (and not by the nonlinear propagation as, for example, thefilamentation). The generation of spatial gradients can be achieved bythe optical systems described herein, while the generation of thetemporal gradients can be achieved by pulse trains or pulse shaping.

Generally, a scaling of the intensity distribution of a beam shape canbe achieved by the imaging ratio of the system, in particular by thefocal length and the numerical aperture of the near field optics of theimaging system. Additional possibilities for scaling result from the useof an additional lens as well as the shifting of the beam shapingelement and/or the far field optics (see the description in connectionwith FIGS. 17 and 22). Thus, the lateral and longitudinal extent of thebeam profile within the work-piece can be influenced. In addition,spatial filters and apertures may be used within the beam path for beamshaping, in order to prepare the beam.

Exemplary laser beam parameters for, for example, ultrashort pulse lasersystems and parameters of the optical system and the elongated focalzone, which can be applied within the range of this disclosure, are:

-   -   pulse energy Ep: 1 μJ to 10 mJ (e.g., 20 μJ to 1000 μJ),    -   energy of a pulse group Eg: 1 μJ to 10 mJ    -   ranges of wavelength: IR, VIS, UV (e.g., 2 μm>λ>200 nm; e.g.,        1550 nm, 1064 nm, 1030 nm, 515 nm, 343 nm)    -   pulse duration (FWHM): 10 fs to 50 ns (e.g., 200 fs to 20 ns)    -   interaction duration (depending on the feed velocity): smaller        100 ns (e.g., 5 ps-15 ns)    -   duty cycle (interaction duration to repetition time of the laser        pulse/the pulse group): less than or equal to 5%, e.g., less        than or equal to 1%    -   raw beam diameter D (1/e2) when entering the optical system:        e.g., in the range from 1 mm to 25 mm    -   focal lengths of the near field optics: 3 mm to 100 mm (e.g., 10        mm to 20 mm)    -   numerical aperture NA of the near field optics: 0.15≦NA≦0.5    -   length of beam profile within the material: larger 20 μm    -   maximal lateral extent of the beam profile within the material,        where applicable in the short direction: smaller 20 λ    -   aspect ratio: larger 20    -   modulation in propagation direction: larger 10 periods over the        focus zone    -   feed dv between two neighboring modifications e.g., for        separating applications:    -   100 nm<dv<10*lateral extent in feed direction    -   feed during interaction duration: e.g., smaller 5% of the        lateral extent in feed direction

Thus, the pulse duration of the laser pulse and the interaction durationrelate to a temporal range, within which, for example, a group of laserpulses interacts with the material for the formation of a singlemodification at a location. Thereby, the interaction duration is shortregarding the present feed velocity, so that all laser pulses of a groupcontribute to a modification at one position.

If the work-piece is thinner than the focus zone is long, the focus zoneis positioned partially outside of the work-piece, so that modificationsmay be caused that are shorter than the focus zone. Such a situation maybe advantageously used to make the processing process robust also withrespect to varying the distance between the optics and the work-piece.In some embodiments, a modification may be advantageous that does notreach through the complete work-piece. In particular, the length of thefocus zone and/or its position within the work-piece may be adapted. Ingeneral, it is noted, that, due to different thresholds for thenonlinear absorption, a focus zone with assumed identical intensity maycause differently large modifications in differing materials.

The aspect ratio relates to the geometry of the beam profile (the focuszone) within the to be processed material as well as the geometry of themodification created with a beam profile. For asymmetric or in lateraldirection modulated (for example, non-rotationally symmetric orring-shaped) beam profiles, the aspect ratio is given by the ratio ofthe length of the modification with respect to a maximum lateral extentin the shortest direction that is present within that range of length.If the beam profile thereby includes a modulation in lateral direction,for example, for ring-shaped beam profiles, then the aspect ratiorelates to the width of a maximum, for a ring-shaped beam profile, forexample, to the strength of the ring. When a plurality of modificationvolumes, which are displaced in lateral direction, are formed, theaspect ratio relates to the lateral extent of a single modification. Fora beam profile modulated in propagation direction (e.g., due tointerferences), the aspect ratio relates to the higher ranking totallength.

Assuming a distance d between the beam shaping element and the focusinglens (near field optics), which is in particular larger than the focallength fN of the near field optics, and an NA of the near field opticswith respect to air >0.15, the used angular spectrum α of the beamshaping element can be in the range tan(α)<f*NA/d<NA/2 and preferablytan(α)>f*NA/(d*4).

The previously mentioned ranges for parameters may allow the processingof a material thickness up to, for example, 5 mm and more (typically 100μm to 1.1 mm) with roughness of the cutting-edge Ra, for example,smaller than 1 μm.

Optical system 1 may further include a beam processing unit 13 foradapting beam parameters such as beam diameter, input intensity profile,input divergence, and/or polarization of laser beam 3. For example, thelaser beam of a pulsed laser system is coupled into optical system 1with, for example, a beam diameter of 5 mm, pulse duration of 6 ps atwavelengths around 1030 nm and is led to processing unit 13.

FIG. 3 shows the schematic setup of optical system 1 for explaining thefunctionality. Optical system 1 is based on a beam shaping element 31and an imaging system 33. Beam shaping element 31 is adapted to receivelaser beam 3. Accordingly, it is adapted to a transverse input intensityprofile 41 of laser beam 3. In addition, beam shaping element 31 isadapted to impose onto laser beam 3 a beam shaping phase distribution 43(schematically indicated by dashes in FIG. 1) over transverse inputintensity profile 41. Imposed phase distribution 43 is such that avirtual optical image 53 (essentially) of elongated focus zone 7 isattributed to laser beam 3, virtual optical image 53 being located infront of beam shaping element 31. Beam shaping element 31 creates inthis manner a virtual beam profile that is located upstream of beamshaping element 31, but does not correspond to the real path of the beambeing at that position.

Imaging system 33 is construed such that the virtual beam profile isimaged into the area of the laser processing machine, in which thework-piece is positioned during the processing. In FIG. 3, imagingsystem 33 includes for that purpose and in beam direction, firstfocusing element, which is referred to herein as far field optics 33A,and an, in direction of the beam, second focusing element, which isreferred to herein as near field optics 33B.

Far field optics 33A is provided in the area of phase imposing and isillustrated in FIG. 3 exemplarily downwards of beam shaping element 31by a lens shape. As will be explained in the following, far field optics33A may also be arranged shortly before beam shaping element 31,composed of components before and after the beam shaping element, and/orcompletely or partially integrated in the same.

After the imposing of the phase within beam shaping element 31, laserbeam 3 propagates in accordance with imaging system 33 over a beamshaping distance Dp to near field optics 33B. Beam shaping distance Dpcorresponds thereby to a propagation length of the laser beam 3, withinwhich imposed phase distribution 43 transforms the transverse inputintensity profile 41 into a transverse output intensity profile 51 atnear field optics 33B. Herein, output intensity profile 51 includesthose transverse intensity profiles in the optical system that aredetermined by the phase imposing. This is usually completed at thelatest in the area of the focal length before the near field optics orwithin the area of the near field optics.

For implementing the concept of a virtual beam profile, there are thefollowing considerations for the propagation length (from beam shapingelement 31 to near field optics 33B), which laser beam 3 has topropagate within the optical system. In general, the optical systemforms an imaging system 33 with a far field focusing action and a nearfield focusing action. The latter is determined by near field optics 33Band thereby by near field focal length fN. The first is determined by afar field focusing action and a respective far field focal length fF.Far field focal length fF can be realized by the separate far fieldoptics 33A and/or can be integrated into the beam shaping element. Seein this respect also FIG. 20. Imaging system 33 has an imaging ratio ofX to 1, whereby X for a demagnification of the virtual image usually islarger than 1. For example, imaging ratios are implemented that arelarger than or equal to 1:1 such as larger than or equal to 5:1, 10:1,20:1, or 40:1. In other words, with this definition of the imaging, thefactor X resembles the magnification of the lateral size of the focuszone into the virtual profile. The angle is respectively demagnified.Attention should be paid to the fact that the imaging ratio goesquadratic into the length of the profile. Accordingly, the longitudinallength of a virtual image becomes smaller, for example, for an imagingratio of 10:1 by a factor of 100 and for an imaging ratio of 20:1 by afactor of 400.

At an imaging ratio of 1:1, there is fN=fF, an overlapping alignment ofthe focal planes is assumed. In general, there is fF=X fN. If the farfield optics 33A is integrated into the beam shaping element, it ispositioned, e.g., at a distance fN+fF from the near field optics, i.e.,typically in the range of the sum of the focal lengths of both opticalelements. For a 1:1 or a de-magnifying imaging system, the propagationlength corresponds therefore at least to twice the focal length of thenear field optics.

Separating far field optics 33A and beam shaping element 31 andassuming, that the virtual optical image should not overlap (inparticular not within the intensity region being relevant for the focuszone) with the beam shaping element, the beam shaping element isarranged at at least at a distance of I/2 downward of the longitudinalcenter of virtual beam profile 53. Here, the length I is thelongitudinal extent of virtual beam profile 53 with respect to therelevant intensity area. The longitudinal center of virtual beam profile53 is located e.g., at the entrance side focal plane of far field optics33A, which is located at a distance fN+fF from near field optics 33B. Inthis case, the propagation length is d=fN+2fF−I/2=(1+2X) fN−I/2,therefore smaller than fN+2fF=(1+2X) fN, or, in other words, smallerthan the distance between the optical elements plus

For the distance fN+2fF=(1+2X) fN, also for increasing beam enlargementsa respectively increasing length I of virtual beam profile 53 can beimaged, whereby—as explained later—a defined end of the profile can bemaintained.

In general, it is mentioned that, due to raw beam divergences andconvergences as well as for deviating adjustment of the imaging system,deviations from the above considerations may occur. In contrast to acomparable image of a real intensity enhancement, i.e., images withcomparable imaging ratios, the beam shaping element is located closer(see the respective discussion on FIGS. 7 and 8). A common distancetherefore lies in a range (1+2X) fN≧d≧2fN.

Due to the imposed phase, transverse output intensity profile 51includes, in comparison to input intensity profile 41, at least onelocal maximum 49 located outside of a beam axis 45. Local maximum 49being located outside beam axis 45 results in a lateral energy entryinto focus zone 7. Depending on beam shaping element 31, local maximum49 of transverse output intensity profile 51 can be made rotationallysymmetric with respect to beam axis 45—as indicated in FIG. 3 in the cutview—or it can be formed in an azimuthal angular range (see, e.g., FIGS.29 and 30). Usually, the beam axis is defined by the center of gravityof the beam of the lateral beam profile. The optical system can usuallybe related to an optical axis, which usually runs through a symmetrypoint of the beam shaping element (e.g., through the center of the DOEor the tip of the reflective hollow cone axicon). For rotationallysymmetric beams and a respective exact alignment, the beam axis maycoincide with the optical axis of the optical system at least insections.

The local maximum can be considered a generic feature of outputintensity profile 51, where in particular for inverse quasi-Bessel beamshapes, a typical substructure with a steep and slowly falling flank canbe formed. That substructure can invert itself due to the focusingaction of the beam forming element and/or the far field optics in therange of an associated far field focal plane. In particular, the outputintensity profile can show within the range of that far field focalplane the local maximum particularly “sharp” or, for example, forinverse quasi-Bessel beam shapes, the local maximum can form itselfquite fast after the beam forming element. However, the aspects of thesubstructure may vary due to the various possibilities in the phaseimposing.

The concept of a virtual beam profile can, on the one side, reduce theconstructional length of optical system 1 and, on the other side, it canavoid the formation of an elongated beam profile with significantintensity enhancement within optical system 1. Imaging system 33 isconfigured such that, within optical system 1, the far field of thevirtual beam profile is formed and that the focusing in the near fieldoptics 33B can be done using a common focusing component such as a lens,a mirror, a microscopic objective, or a combination thereof. In thatcase, “common” is understood herein in the sense of that thecharacteristic beam shape is essentially imposed by beam shaping element31 and not by near field optics 33B.

In FIG. 3, a path of the beam is indicated for illustration thatcorresponds to a beam herein referred to as an inverse quasi-Besselbeam. For that purpose, the path of the beam is illustrated downstreamof beam shaping element 31 with solid lines. Upstream of beam shapingelement 31, instead of incident collimated beam 3, the virtual beamprofile is sketched in analogy to a real quasi-Bessel beam by dashedlines.

Similar to a common quasi-Bessel beam, also the inverse quasi-Besselbeam has a ring structure in the focal plane of far field optics 33A.However, divergent beam areas 55A, 55B indicated in the schematic cutview, which impinge on far field optics 33A, do not result from a “real”quasi-Bessel beam profile, but they result directly from the interactionof beam shaping element 31 with incident laser beam 3. Due to the directinteraction, beam areas 55A, 55B are shaped in their lateral intensitydistribution by transverse beam profile 41 of laser beam 3. Accordingly,for a Gaussian input beam, the intensity decreases in the radialdirection principally in beam areas 55A, 55B from the inside to theoutside. Due to the divergence of beam areas 55A, 55B, typically an areaof low (in the ideal case no) intensity is formed accordingly on thebeam axis for the phase-modulated beam portions. In that case, thedivergence of a beam portion, accordingly also a divergent beam portion,relates herein to a beam portion that moves away from the beam axis.However, in that area, a beam portion of a phase unmodulated beam and/oralso an additional, phase-modulated beam portion may be superimposed.With respect to the development of the beam within the optical systemduring the shaping of an inverse

Bessel beam, it is referred to the description of FIGS. 33 and 34. Thisintensity behavior is schematically indicated in transverse intensitycourses (e.g., transverse intensity beam profile segments) 57A and 57B.It is noted that the intensity courses along the propagation length canchange due to imposed phase distribution 43. At least, however, withinthe initial area (i.e., beam areas 55A, 55B laying close to beam shapingelement 31) and due to the beam shaping element 31 acting in general asa pure phase mask, the incident intensity profile of laser beam 3dominates the divergent phase-modulated beam portions.

For a clear explanation of an inverse quasi-Bessel beam, furtherintensity courses 57A′ and 57B′ are schematically indicated in FIG. 3.Here, it is assumed that beam shaping element 31 influences only thephase and not the amplitude. One recognizes that the focusing by farfield optics 33A (or the respective far field action of beam shapingelement 31) reverses the intensity course at the exit of optical system1 such that, during the formation of elongated focus zone 7 on beam axis45, at first low intensities are superposed, which originate from thedecreasing flanks of the incident Gaussian beam profile. Thereafter, thehigher intensities superpose, which originate from the central area ofthe incident Gaussian beam profile. In this context it is noted that notonly the intensity on the beam shaping element, but also thecontributing area has to be acknowledged. For rotationally symmetry, thedistance enters accordingly quadratic. As in particular illustrated inconnection with FIG. 4, the longitudinal intensity profile ends exactlyin that area, in which the beam portions from the center of the inputprofile cross. In the center, although the highest intensity is present,the area goes to zero. Moreover, it is noted that, after the focus zone,a reversed intensity course is present again, which corresponds tointensity courses 57A, 57B after the beam shaping element (assuming nointeraction with a material).

Due to imaging with imaging system 33, there are incident virtualintensity courses 57A″ and 57B″, which are accordingly schematicallyindicated with respect to the virtual beam shaping in FIG. 3. Thosecorrespond in principle to intensity courses 57A′ and 57B′.

Those intensity courses, which are inverted in comparison to aquasi-Bessel beam, cause a specific longitudinal intensity course forthe inverse quasi-Bessel beam for focus zone 7 as well as in the virtualbeam profile, i.e., optical image 53, because here the superposition ofbeam portions 55A, 55B is done virtually. For the respective discussionof the intensity course for a conventional quasi-Bessel beam, it isreferred to FIGS. 7 and 8 and the respective description.

FIG. 4 exemplarily illustrates a longitudinal intensity distribution 61within elongated focus zone 7 as it can be calculated for the imaging ofvirtual optical image 53 of an inverse quasi-Bessel beam shape. Depictedis a normed intensity I in Z direction. It is noted that a propagationdirection according to a normal incidence (in Z direction) onto material9 is not required and, as illustrated in connection with FIG. 2, cantake place alternatively under an angle with respect to the Z direction.

One recognizes in FIG. 4 an initially slow intensity increase 61A overseveral 100 micrometer (initial superposition of low (outer)intensities) up to an intensity maximum, followed by a strong intensitydecrease 61B (superposition of the high (central) intensities). For aninverse Bessel beam shape, the result is therefore a hard border of thelongitudinal intensity distribution in propagation direction (the Zdirection in FIG. 4). As one can in particular recognize in view ofintensity courses 57A′ and 57B′ shown in FIG. 3, the hard border isbased on the fact that the end of longitudinal intensity distribution 61relies on the contributions of the beam center of the incident laserbeam having admittedly a lot of intensity, but on a strongly reduced(going to zero) area. In other words, the end relies on the imaging of avirtual beam profile in which in the center for the inverse quasi-Besselbeam a hole is created. The strong gradient at the intensity decrease atthe end relies on the high intensity in the center of the input profile,limited, however, by the disappearing area. For an ideal imaging system,the longitudinal extent of intensity distribution 61 is defined by theposition of the virtual profile and the imaging scale. If in additionthe work-piece includes a large refractive index, the beam profile isaccordingly lengthened.

In this context it is added that the hard border has the consequence inlaser processing machines that the, in propagation direction, front endof a modification is essentially stationary in propagation directionalso if the incident transverse beam profile is increased. Themodification changes its extent only in the back part, i.e., it canlengthen in direction to the near field optics, if the input beamdiameter of the laser beam enlarges. A once set position of the hardborder with respect to the work-piece support or the work-piece itselfcan thereby avoid high intensities downstream of the modification. Incontrast thereto, an enlargement of the input beam diameter, whenimaging a real intensity enhancement, causes an elongation of themodification in propagation direction, i.e., for example into awork-piece support, which can result in damages of the same.

FIG. 5 shows an exemplary X-Y-cut 63 of the intensity within focus zone7 for the longitudinal intensity distribution 61 shown in FIG. 4. It isnoted that herein some grayscale illustrations such as FIGS. 5, 30, 31,40, 45, 46, and 50 are based on a color illustration so that maximumvalues of the intensity/amplitude can be illustrated dark. For example,the center of focus zone 7 (highest intensity) in FIG. 5 is shown darkand is surrounded by a brighter area of lower intensity. The sameapplies, for example, to focus zone 707 in FIGS. 30 and 31 and for focuszones 1007A and 1007B in FIG. 50. One recognizes the elongated formationof focus zone 7 over several hundred micrometers at a transverse extentof some few micrometers. Together with the threshold behavior of thenonlinear absorption, such a beam profile can cause a clearly definedelongated modification within the work-piece. The elongated shape offocus zone 7 includes, for example, an aspect ratio, i.e., the ratio ofthe length of the focus zone to a maximal extent in the lateral shortestdirection being present within that length—the latter fornon-rotationally symmetric profiles, in the range from 10:1 to 1000:1,e.g., 20:1 or more, for example 50:1 to 400:1.

If one frees oneself from the beam shape—shown in FIG. 4—of an inversequasi-Bessel beam, which is not modified in propagation direction withrespect to amplitude, beam shaping element 31 can additionally create anamplitude redistribution in the far field, which e.g., can be used foran intensity modification in propagation direction. However, the therebycreated intensity distribution in front of focus zone 7 can no longer bepresented in a very clear manner. Nevertheless, often initial stages ofinversions will show up in the beginning region or in the end region ofthe longitudinal intensity profile, for example a slow increase and asteep decrease. However, a (phase caused) amplitude redistribution bythe phase description of beam shaping element 31 may just exactly be setto an inverted intensity distribution, in order to cause, for example, aform of a longitudinal flat top intensity profile.

Additionally, the following feature for distinguishing from a “real”beam shape may be maintained: For the case of a real Gaussian inputbeam, there exists, e.g., for a real Axicon, a plane between near fieldoptics and focus zone at which the demagnified Gaussian transverse beamprofile of the input beam is present and can accordingly be madevisible. A respective imaging exists for the virtual optical image.However, in this case, the image plane, in which the demagnifiedGaussian transverse beam profile is present, lies behind the focus zone.The transverse beam profile can accordingly be made visible. Thisapplies generally to phase masks for the herein disclosed inverse beamshapes, if those are illuminated with a Gaussian beam profile.Specifically, the demagnified Gaussian transverse beam profile ispositioned in the image plane of the beam shaping element and thereforeusually directly downstream of the focus zone. Due to the alreadyperformed divergence, it is therefore significantly larger than thetransverse beam profile of the inverse quasi-Bessel beam in the focuszone. Also, it is much lower in the intensity.

One can recognize the position of the imaged Gaussian transverse beamprofile of the input beam by a fast turn up of the structure of the beamprofile, i.e., a strong change over a small lateral area. For example,the transverse intensity profile of the inverse quasi-Bessel beam ispresent in the focus zone. When passing through the image plane of thebeam shaping element, then “quasi” immediately the dark spot in thecenter is formed. For an inverse quasi-Bessel beam, this is different atthe beginning of the focus zone. There, due to the increasedsuperposition of the border areas of the Gaussian beam profile, a slowtransition is made from a dark center to the transverse intensityprofile of the inverse quasi-Bessel beam, which is filled in the center.In other words, in longitudinal direction, the intensity increases overa larger area then it decreases at the end. At the end, that transitionis accordingly clearly sharply limited. It is added that, when imaging areal Bessel intensity enhancement, the behavior at the end and thebehavior at the beginning are interchanged, i.e., at the end of theBessel beam profile, the dark spot forms more slowly.

As previously explained, the concept of using a virtual beam profiletherefore has an effect inter alia on the phase imposing to be appliedand the resulting intensity courses in focus zone 7.

FIG. 6 illustrates modification zones 65 that were created in thecontext of an experimental study for examining the formation ofmodifications in a transparent material. Each modification zone 65 goesback to the interaction with a group of laser pulses, for example two 6ps pulses at a temporal separation of about 14 ns. The shape of themodification zones corresponds to the shape of elongated focus zone 7 asassumed in accordance with FIGS. 4 and 5. The maximal length is limitedby the geometry of elongated focus zone 7 at a requiredintensity/fluence.

The upper four images illustrate the threshold behavior for pulse groupenergies Eg from about 200 to 40 μJ. The lower four images illustratethe shaping of the elongated modification zones 65 at pulse groupenergies Eg from about 30 μJ to 200 μJ. With increasing total energy Eg,the modification zone lengthens in the direction of the beam entrance(near field optics), because the threshold intensity for the nonlinearabsorption is reached within a longer area of focus zone 7. The end ofthe modification in beam propagation direction is in its positionessentially stationary, and even in particular without secondarycorrection of the distance of a near field optics (33B) to the to beprocessed work-piece. At lower energies, an initial walk in beamdirection of the back end may occur due to the existing gradient inlongitudinal direction, in particular if the modification threshold liesat small intensities within the beam profile. However, the walkdecreases at medium and high energies, because the generation of theinverse quasi-Bessel beam profile includes in propagation direction animplicit maximal back end.

A similar behavior in the change of the longitudinal extent of themodification is also created for a radially increasing beam diameter ofincident laser beam 3. Also in that case, the modification zone islengthening in direction of the beam entrance (near field optics),because the intensity areas of incident laser beam 3, which are added ina radial direction at the outside, guide energy into the longitudinalintensity area in the area of slow intensity increase 61A (i.e.,intensity increase with slow gradient). The maximum of the intensitydistribution will accordingly be shifted in direction of the beamentrance. The end of the modification in beam propagation direction isin contrast in its position essentially stationary, because thatposition is supplied with energy by the center of the beam of incidentlaser beam 3. In addition, it is noted that this behavior can beobserved also for modified inverse quasi-Bessel beam shapes. Forexample, for a flat top beam shape as discussed in connection with FIGS.23 to 26, the position of the end of the modification would essentiallynot change for a change in the beam diameter. For such a changedincident intensity profile, the beam shaping element may furthereventually no longer result in an optimized flat top structure so thatthis may result in modulations in the intensity and eventually avariation of the beginning.

FIG. 7 serves as an illustration of a beam guidance at which a realintensity enhancement 71 is generated by a beam shaping optics 73 suchas an axicon. This corresponds to the known formation of a quasi-Besselbeam. Intensity enhancement 71 is then imaged by a telescope 75 intowork-piece 9 by forming a focus zone 77. As shown in FIG. 7, in such asetup, there is the danger that the real intensity enhancement 71damages a far field optics 79 of telescope system 75, in particular if asmall setup length is to be realized. The herein disclosed opticalsystem (see, e.g., FIG. 3), which implements the concept of a virtualimage, bypasses that risk of a damage to the beam guiding optics.

FIG. 8 illustrates for completeness a longitudinal intensitydistribution 81 in Z direction that results from the setup of FIG. 7.After an ab initio steep increase 81A, an intensity maximum is reached,beginning at which the intensity decreases again. At lower intensities,a slowly vanishing drop 81B (vanishing drop of small gradient) begins.One sees the general reversal of the longitudinal intensitydistributions 61 and 68 of FIGS. 4 and 8, at which the “hard border” atthe end is replaced by a “hard beginning”.

For such a quasi-Bessel beam, the passing through an axicon with a laserbeam having an incident Gaussian beam profile 83 will result insuperposed beam portions 85A, 85B, the intensity weights of which resultin real longitudinal intensity distribution 81 (at first superpositionof the intensities of the central area of Gaussian beam profile 83, thensuperposition of lower (outer) intensities of Gaussian beam profile 83).For explaining, again schematic intensity courses 87A and 87B areindicated downstream of far field optics 79, and intensity courses 87A′and 87B′ are indicated upstream of focus zone 77.

In the following, various exemplary configurations of optical systemsare explained that implement the concept of virtual intensityenhancement. They include beam shaping elements in the transmission andreflection, wherein the imposing of the phase distribution is performedin particularly refractive, reflective, or diffractive. It is referredto the preceding description with respect to the already describedcomponents such as laser system 11.

In view of the distances of beam shaping optics 73 from the near fieldoptics, the following values can apply similar to the considerations forthe virtual image. For a real beam profile, one would typically positionthe center of the to be imaged real beam profile of length I in theentrance-side focal length of the far field optics. A typical distancewould then be at least

fN+2fF+I/2=(1+2X) fN+I/2, thus larger than fN+2fF, in other words,larger than the distance between the optical elements plus

FIG. 9 shows a refractive beam shaping with the help of a hollow coneaxicon 131A. This creates a virtual inverse quasi-Bessel beam profile153A upward of hollow cone axicon 131A. The same is indicated in FIG. 9by dashed lines, a real intensity enhancement is not present in thatarea. In addition, in the embodiment of FIG. 9, the far field optics isconfigured in beam propagation direction downstream of hollow coneaxicon 131A as plano-convex lens 133A. Near field optics 33B causes thefocusing of the laser beam into focus zone 7, so that the virtualinverse quasi-Bessel beam profile 153A is related to the laser beam asvirtual optical image of focus zone 7.

FIG. 10 shows an embodiment with a hollow cone axicon lens system 131Bthat is used as a refractive beam shaping element. Here, the far fieldoptics is integrated in the beam shaping element as convex lens surface133B, which is positioned at the entrance side of the hollow coneaxicon. This setup creates similarly a virtual inverse quasi-Bessel beamprofile 153B.

FIG. 11A illustrates an embodiment with a reflective beam shapingelement, in particular a reflective axicon mirror system 131C. A highlyreflective surface of the beam shaping element is shaped such that thebeam shaping feature of a reflective axicon is combined with the farfield forming component of a focusing hollow mirror. Accordingly, axiconmirror system 131C has the function of beam shaping as well as of thefar field optics. A virtual inverse quasi-Bessel beam profile 153C isindicated at the backside of axicon mirror system 131C, thus in an areathat is not passed by laser beam 3.

As is further shown in FIG. 11A, after beam adaptation unit 13, laserbeam 3 of laser system 11 is coupled into optical system 1 by adeflection mirror 140. Deflection mirror 140 is, for example, arrangedon the optical axis between axicon mirror system 131C and near fieldoptics 33B and guides the beam to beam shaping element 131C. In someembodiments, the deflection mirror may, for example, be centrallydrilled through to guide as less as possible light onto the central areaof beam shaping element 131C, which potentially has optical errors. Inaddition to those aspects of filtering described in the following inconnection with FIGS. 17 and 18, it is already noted at this stage thatdeflection mirror 140 at the same time blocks an undesired central beamportion such that the same is not focused by near field optics 33B.

FIG. 11B shows a further embodiment of an optical system based on areflective beam shaping element. The beam shaping element in form ofreflective axicon mirror system 131C is illuminated thereby with laserbeam 3 through an opening 141 of a drilled through deflection mirror140′. That reflected and phase imposed beam impinges then after theformation of a e.g., ring-shaped far field onto deflection mirror 140′.The same guides the beam onto near field optics 33B for focusing intothe elongated focus zone. The opening serves accordingly in addition askind of a filter/diaphragm of the central area of the reflected beam.

In another embodiment with a reflective beam shaping element, theoptical system includes a reflective axicon, a drilled throughoff-axis-parabolic mirror, and the near field optics. That reflectiveaxicon includes for the beam shaping a conical grinded based body, theconical surface of which is coated highly reflective. The laser beam canbe irradiated through the opening in the off-axis-parabolic mirror ontothe reflective axicon. The reflected and beam shaped beam impinges thenon the off-axis-parabolic mirror that redirects the beam on near fieldoptics 33B and at the same time collimates the same.

FIGS. 12 and 13 show embodiments of the optical system with digitalizedbeam shaping elements. Here, the digitalization can relate to the use ofdiscrete values for the phase shift and/or the lateral structure (forexample, pixel structure). The use of spatial light modulators (SLMs) isone of many different possibilities to realize the beam shaping withprogrammable or also permanently written diffractive optical elements(DOE).

In addition to the simple generation of one or more virtual beamprofiles, e.g., according to the phase imposing of one or more hollowcone axicons, diffractive optical elements allow the desiredmodification, for example, for homogenizing of the longitudinalintensity distribution. For this, deviations in the phase canexemplarily be used in the range equal to or smaller than 50%, e.g.,equal to or smaller than 20% or equal to or smaller than 10% withrespect to, for example, the hollow cone axicon phase (and thereby of aninverse quasi-Bessel beam). In general, SLMs allow very fine phasechanges at a lateral rough resolution, in contrast to, for example,lithographically generated, permanently written DOEs. Permanentlywritten DOEs include e.g., plano-parallel steps, the thickness of whichdetermine the phase. So, the lithographic manufacturing allows a largelateral resolution. Binary steps can result in real and virtual beamprofiles. Only a number of more than two phase steps can result in adifferentiation in the sense of a preferred direction for the virtualbeam profile. For example, four or eight or more phase steps allow anefficient beam shaping with respect to the virtual beam profile.However, the discretization can cause secondary orders that can, forexample, be filtered out. Manufacturing methods for continuousmicrostructures include, for example, the analog-lithography or thenanoimprint-lithography.

Herein, the structural element of a diffractive optical beam shapingelement, which causes the phase imposing and is configured in an arealshape, be it an adjustable SLM or a permanently written DOE, is referredto as a phase mask. Depending on the type of configuration of the DOE,it may be used in transmission or in reflection to impose a phasedistribution on a laser beam.

In FIG. 12, a spatial light modulator 31A is used in reflection forphase imposing. For example, spatial light modulator 31A is based on a“liquid crystal on silicon” (LCOS) that enables a phase shift that isprogrammable for the individual pixels. Spatial light modulators canfurther be based on micro-systems (MEMS), micro-optoelectro-mechanicalsystems (MOEMS), or micro-mirror-matrix systems. In SLMs, the pixelscan, for example, be controlled electronically to cause a specific phaseimposing over the transverse input intensity profile. The electroniccontrollability allows, for example, the online-setting of phases and,thus, the adaptation of focus zone 7, e.g., in dependence of the to beprocessed material or in reaction of fluctuations of the laser. In theconfiguration of FIG. 12, the function of a diffractive axicon for thegeneration of a virtual inverse quasi-Bessel beam profile can becombined, for example, with the far field forming action of a far fieldoptics by the phase shifting of the spatial light modulator 31A.Alternatively, a permanently written reflective DOE can be used as beamshaping element 31A.

FIG. 13 is a schematic view of an optical system based on a DOE 31B, forwhich the phase imposing is permanently written in DOE 31B. DOE 31B isused in that case in transmission. As in FIG. 12, the phase shift,which, for example, results in a virtual quasi-Bessel beam profile, aswell as the focusing property of far field optics are combined withinthe DOE 31B.

The optical systems of FIGS. 9 to 13 can result in output intensityprofiles that correspond to inverse quasi-Bessel beam profiles and thatare attributed to virtual optical images.

FIG. 14 illustrates an example of a phase distribution 243 as it can beprovided e.g., in DOE 31B. Phase distribution 243 is rotationallysymmetric. One recognizes ring-shaped phase distributions, the frequencyof which is modulated in radial direction. The rings point to thegeneration of a rotationally symmetric virtual quasi-Bessel beamprofile. The frequency modulation points to the integration of the phasecomponent of the far field optics in the phase distribution for beamshaping. In FIG. 14, the phases are indicated in the range of ±π. Inalternative configurations, discrete phase distributions such as binaryphase distributions or multi-step (for example, 4 or more levels in therange of the phase shift from 0 to 2π) phase distributions can beimplemented in DOE phase masks.

FIGS. 15 and 16 illustrate exemplarily an output intensity profile 251within the intensity cross-section (FIG. 15) and in the 2D-top view(FIG. 16). One recognizes an intensity maximum 249 extending in a ringshape around beam axis 45. There is hardly any intensity in the beamcenter.

In some embodiments, the transition into the inverse quasi-Bessel beamwill not be complete such that accordingly a non-phase-modulatedremaining beam, for example with a Gaussian beam shape, is superposed tothe ring-shaped intensity profile. FIG. 15 indicates schematically sucha non-phase-modulated beam portion 252 by a dash-dotted line.

Maximum 249 of that intensity distribution in FIG. 15 is an example of alocal intensity maximum, with which an original input intensity profile(e.g., a Gaussian beam profile) was modified in the area of thetransverse output intensity profile. The rotational symmetry of the ringstructure is due to the rotational symmetry of the inverse quasi-Besselbeam profile. In alternative embodiments, the local intensity maximum islimited to an azimuthal angular range. In addition, a superposition ofazimuthal limited and/or ring-shaped local maxima may be given.

When using a refractive hollow cone axicon (see FIGS. 9 and 10) for thegeneration of an inverse quasi-Bessel beam-shaped output intensityprofile, undesired beam portions may be created under undesired anglesfor a non-perfect tip of the axicon. Also for diffractive beam shapingelements, non-desired beam portions may appear. For example, anon-phase-modulated beam portion, which cannot be neglected, oradditional orders of diffraction in the far field of the laser beam canbe present.

The herein disclosed optical systems simplify, by using the far fieldcomponents, the insertion and the shape selection of filters to filterout such disturbing beam portions. In particular, these undesired beamportions can be separated from the desired beam portions (beam for use)in a simple manner in the area of the Fourier plane.

Referring to the non-phase-modulated beam portion 252 of FIG. 15, FIG.17 shows an exemplary optical system that is based on the optical systemshown in FIG. 3. However, additionally a filtering ofnon-phase-modulated portions is performed in the area of the Fourierplane of imaging system 33. Exemplarily, a spatial filter unit 220 isindicated upward of near field optics 33B in FIG. 17.

Filter unit 220 includes a central area around beam axis 45 that blocks,for example, the Gaussian intensity distribution—indicated in FIG. 15—ofthe non-phase-modulated beam portion 252. Filter unit 220 canadditionally include sections, which are located radially furtheroutside, for blocking higher orders of diffraction by the DOE or theSLM.

In general, filter unit 220 is provided for the suppression ofnon-phase-modulated base modes and higher diffraction orders as well asof scattered radiation of the various herein disclosed refractive,reflective, or diffractive beam shaping elements. For rotationallysymmetric output intensity profiles, usually also the filter unit ismade rotationally symmetric. In some embodiments, only some portions offilter unit 220 or no filtering at all is provided.

Diffractive beam shaping elements allow a further approach forsuppressing the non-phase-modulated beam portions. For this, anadditional phase contribution is imposed to deflect the phase-modulatedbeam portion.

FIG. 18 shows, for example, an optical system in which the diffractiveoptical element 31 is additionally provided with a linear phasecontribution. The linear phase contribution results in a deflection 230of phase-modulated beam 203A. Non-phase-modulated beam portion 203B isnot deflected and impinges, for example, on a filter unit 222.

FIG. 19 shows a further embodiment of an optical system that utilizesthe use of the far field component additionally for the implementationof a scan approach. In general, a scan system allows shifting focus zone7 within a certain range. In general, it is possible by the separationof the beam shape from the near field focusing to provide favorabletelecentric scan approaches, in particular for the volume absorption. Insome embodiments, in addition the location as well as the angle can beset. Accordingly, such scanner systems can allow writing fine contoursinto a work-piece.

In the configuration of FIG. 19, a scanner mirror 310 is positioned atthe image side focal plane of a near field optics 333B. Scanner mirror310 deflects the laser beam in the range of the output intensitydistribution onto near field optics 333B positioned at the side. Thedeflection in the Fourier plane results in that the propagationdirection in the work-piece is preserved despite the displacement inlocation. The scanning region itself is determined by the size of nearfield optics 333B.

If scanner mirror 310 is not correctly positioned in the focal plane ofnear field optics 333B or if it can be moved with respect thereto, thenan orientation of the elongated focus zone, in particular an angulardeviation from the Z direction in FIG. 2, can be set.

With the help of a configuration in accordance with the optical systemshown in FIG. 13, FIG. 20 explains exemplarily the underlying imagingfeatures. The optical system includes a beam shaping element 31 thatoperates also as a far field optics and is therefore characterized by afocal length fF. In addition, the optical system includes near fieldoptics 33B that is characterized by focal length fN. In FIG. 20, thefocal planes of the far field optics and the near field optics coincide.Accordingly, in FIG. 20 only one focal plane 340 is indicated by adashed line. In that configuration of overlapping focal planes, theimaging system images for incidence of a plane wave front generally avirtual beam shape 253 onto elongated focus zone 7, for example, aninverse quasi-Bessel beam profile, inverse modulated or homogenizedquasi-Bessel beam profiles as examples for inverse quasi-Bessel/Airybeam shapes.

Though the focal planes do not need to overlap always. For example, theimaging system can be adapted to a given beam divergence, but laser beam3 may be incident with another divergence. In those cases, still avirtual optical image being positioned in front of the beam shapingelement is attributed to elongated focus zone 7, but it does not need tobe a perfect imaging. A similar situation may be given for an intendedmisalignment of the imaging system, for example, in connection with ascanner device.

FIG. 20 illustrates in addition the terms “far field optics” and “nearfield optics”. The far field optics creates the far field of virtualbeam path 253 in the range of far field focal length fF. As previouslyalready explained, the far field optics can be distributed in itsfunction, for example, be made of one or more components, which arearranged before and/or after the beam shaping element and displaced withrespect to the same, and/or be integrated into the beam shaping element.The near field optics focuses the beam with the smaller focal length fNin the direction of the work-piece and thereby forms the focus zone.Thus, the far field of virtual beam profile 53 with respect to the farfield optics, as well as the far field of focus zone 7 with respect tonear field optics 33B are positioned in the area of focal plane 340.

Also for non-perfect imaging (e.g., non-overlapping focus planes of farfield optics and near field optics), essentially an acceptable intensitydistribution in the focus zone can be given, because the intensityprofile, which impinges onto the near field optics, changes only alittle.

For example, in the case of an inverse quasi-Bessel beam shape, thefirst focusing by the far field optics within the optical system causesan adaptation of the ring size on the near field optics. In that manner,the far field optics has a focusing action onto the ring diameter,which, as indicated in the figures, decreases up to some type ofintermediate focus.

FIG. 21 illustrates the beam path in an optical system for the case thata convergent laser beam 3′ impinges on beam shaping element 31.Phase-modulated portion 303A of the laser beam is focused onto elongatedfocus zone 7. Due to the convergence of incident laser beam 3′ (andeventually due to a separate focusing far field optics or an integrationinto the phase distribution of beam shaping element 31), thenon-phase-modulated portion 303B (dash dotted line) decreases furtherduring the propagation length Dp and impinges on a central area of nearfield optics 33B. Thereby, a focus 350 for non-phase-modulated beamportion 303B is formed that is closer to near field lens 33B than it iselongated focus zone 7. The non-phase-modulated portion is stronglydivergent after focus 350, so that those intensities are no longerreached within the work-piece with respect to the non-phase-modulatedbeam portion 303B that result in nonlinear absorption. In such aconfiguration, one can do without filtering non-phase-modulated beamportions 303B.

Nevertheless, a spatially localized filter unit can be provided in thearea of focus 350 (or even between far field optics and near fieldoptics, if the beam is strongly focused) such that non-phase-modulatedbeam portion 303B is kept out of the interaction zone and thework-piece.

FIG. 22 shows an optical system that is equipped with an additional lens400 upstream of beam shaping element 31. Lens 400—as an example of anadditional focusing component—is located at a distance DA to beamshaping element 31.

Beam shaping element 31 has a phase distribution that is set for aspecific beam diameter. The illuminated part of that beam shapingelement, i.e., the beam diameter of the input intensity profile at beamshaping element 31, can be adapted by the translatability of lens 400with respect to beam shaping unit 31.

In some embodiments, lens 400 can be compensated before beam shapingelement 31 within the phase mask of beam shaping element 31 so that theimaging does not change and only the 0. order, i.e., thenon-phase-modulated, portion is focused.

In general, lens 400 can also be understood as a component of the farfield optics. If the far field optics includes a plurality ofcomponents, which can be translated with respect to each other and withrespect to the near field optics, then the imaging scale can be changedby a suitable translation. In some embodiments, lens 400, the beamshaping element, or both can be translated together to adjust theimaging scale of optical system 1. In some embodiments, lens 400 can beused as a first telescope-part-lens for adapting the beam diameter onthe beam shaping element, whereby a second telescope-part-lens iscalculated into the phase mask.

In some embodiments, lens 400 can be translated to perform a fineadjustment of the raw beam in particular for a longitudinal flat topbeam shape or multi-spot formation.

If the input beam is selected such that a convergent or divergent beamis present at beam shaping element 31, then one can—also in this case inaccordance with FIG. 21 under certain conditions—do not use a filterunit for non-phase-modulated beam portion 403B. I.e., intensities forthe nonlinear absorption within the work-piece are only reached by thephase-modulated beam portion 403A.

Diffractive optical elements allow a digitalized and e.g., pixel basedphase adaptation over the input intensity profile. Starting from theintensity distribution of an inverse quasi-Bessel beam shape, alongitudinal flat top intensity profile can, for example, be generatedin focus zone 7. For that purpose, the phase distribution within thebeam shaping element can be influenced such that intensity contributionsin the output intensity profile are taken out of the area, which formsthe intensity maximum and the tails of the Bessel beam, and are radiallyredistributed by a phase change such that, for the later focusing bynear field optics 33B, the increasing area 61A and the decreasing area61B are magnified or far extending tails are avoided to the most part(e.g., by pushing power from the tails into the homogenized area).

A respective output intensity profile 551 is shown in FIG. 23 (intensitycross-section) and FIG. 24 (2D-top view). One recognizes in theintensity cross-section of FIG. 23 that—in comparison to FIG. 15—thelocal maximum is broadened in the radial direction and modulated. Theresult is a respectively radially extended modulated ring structure 549.

FIG. 25 shows the focusing of such an output intensity distribution 551.The result is a longitudinal quasi-homogenized intensity distribution(flat top) 561 over a range from about 700 μm in Z direction.

In analogy to FIG. 6, FIG. 26 shows modification zones 565(modifications) in a transparent material 9. The upper four imagesillustrate again the threshold behavior for pulse group energies Eg fromabout 20 μJ to 40 μJ, while the lower four images show increasing pulsegroup energies Eg from about 30 μJ to 200 μJ. One recognizes that, whenthe threshold is passed, the modification zones form essentially alwaysover the same range of extent in Z direction within work-piece 9. Thisis based on the essentially constant intensity having only a shortincrease and drop off. With increasing energy, however, not only thestrength but also the lateral extent of the modification zonesincreases.

Another embodiment is shown in FIG. 27, which allows reaching a sequenceof intensity enhancement in propagation direction. In general,supplemental phase imposing can be done in the area of the image sidefocal plane of near field optics 33B such as lateral and/or longitudinalmulti-spot phase imposing. Specifically, one recognizes in FIG. 27 asequence of three intensity maxima 661A, 661B, and 661C, which each havean intensity distribution in accordance with FIG. 4.

This sequence can be generated by a longitudinal multi-spot phaseimposing or the use of a multi-focal lens as near field optics 33B. So,for example, an additional diffractive optical element may be providedin the area of the Fourier plane (focal plane of near field optics 33B)or close to near field optics 33B, which provides an additionalphase-modulation for the three foci. Such phase adaptations are known,for example, from EP 1 212 166 B1, the entire contents of which areincorporated by reference.

In connection with FIGS. 28 to 31, a further potential formation of anelongated focus zone is illustrated for the case of an accelerated Airybeam shape.

FIG. 28 shows a phase distribution 743 as it can be imposed within beamshaping element 31 onto the input intensity profile. Here, facedistribution 743 includes the phase distribution, which is required fora generation of the accelerated beam, and the phase distribution of aconcave lens, which compensates a raw beam convergence. In general, aphase mask of an accelerated beam creates a well collimated beam whichdoes not change significantly over the propagation distance and which isthen focused with the near field component in a so-called acceleratedbeam shape.

FIGS. 29 and 30 illustrate the associated output intensity profile 751in the cut view (FIG. 29) and in the top view (FIG. 30). One recognizesthat the intensity maximum is displaced slightly from the center (i.e.,beside the beam axis 45) in Y direction. Thus, the transverse outputintensity profile 751 is modified with respect to the input intensityprofile with a local maximum 749, which is located outside of beam axis45.

The focusing of such an output intensity profile 751 results inelongated and curved focus zone 707 that is illustrated in FIG. 31.Thereby it is allowed that such an accelerated beam profile can be usedalso in combination with non-transparent media, if the focus zone isguided, for example, in Y direction to the border of such a material.The resulting interaction would, for example, provide a rounding of theside of the material. In other embodiments, such a beam profile can beused with transparent materials for cutting with curved cutting faces.

In some embodiments, an optical system is configured, for example, suchthat a real intensity enhancement in accordance with FIG. 7 as well as avirtual intensity enhancement in accordance with FIG. 3 is created.Thereby, the longitudinal extent of modification zones can be widened.

FIG. 32 shows schematically an exemplary optical system with a binaryDOE 31C. If a laser beam 3 falls onto binary DOE 31C, on the one hand, areal intensity enhancement 871 is formed, for example, a quasi-Besselbeam downstream of DOE 31C.

On the other hand, a beam portion is formed, which is associated with avirtual image 853—downstream of DOE 31C—of an elongated focus zone 807A,for example, in the shape of an inverse quasi-Bessel beam.

The optical system includes further a telescope system 833 with a farfield optics 833A and a near field optics 833B. Telescope system 833images virtual image 853 as well as real intensity enhancement 871 intothe to be processed material 9. For that purpose, binary DOE 31C ispositioned in or close to the focal plane of far field optics 833A.

The imaging results in an enlarged interaction region that includeselongated focus zone 807A and focus zone 807B that originates from thereal intensity enhancement 871. In the resulting sequence of successivefocus zones 807A and 807B, the intensity for (inverse) quasi-Besselbeams is at first in accordance with the intensity distribution shown inFIG. 4 and there-after in accordance with the intensity distributionshown in FIG. 8. The result is an intensity distribution with a lowintensity intermediate space that is formed by the strong intensity drop61B and the strong intensity raise 81A. That low intensity intermediatespace can, for example, be provided in the region of a contact zone whenprocessing a pair of on each other lying work-pieces. In addition, thisapproach allows that one can achieve twice the length for theinteraction for identical input beam diameter and identical angularrange, which is covered by the optical system.

In some embodiments, the non-phase-modulated portion can be focused inthe area between the successive focus zones 807A and 807B. A respectiveGaussian focus 807C is additionally shown schematically in FIG. 32. Insuch an embodiment, an adaptation of the efficiency of the diffractionmay become possible, because the non-phase-modulated beam may be usedfor filling intensity voids.

Herein, some aspects were described exemplarily based on selectedvirtual beam profiles. In general, those aspects can be transferred ontothe herein as (inverse) virtual beam shapes described types of beamssuch as inverse quasi-Bessel/Airy beam shapes, e.g., inversequasi-Bessel beam profiles or inverse modulated or homogenizedquasi-Bessel beam profiles.

In connection with FIGS. 33A to 33D and 34, the propagation from thebeam shaping element to the near field optics is explained by beamprofiles and amplitude courses for an inverse quasi-Bessel beam. Lightergrayscale values correspond to larger amplitudes. A respective invertedquasi-Bessel beam can be generated with the herein disclosed refractive,reflective, and diffractive optical systems, for example, with thehollow cone axicon systems and the DOE systems. A DOE system can bebased, for example, on the phase distribution of a phase mask shown inFIG. 14, in which a focusing phase contribution is provided in additionto the phase required for the inverse quasi-Bessel beam.

It is assumed that a laser beam having a rotationally symmetric Gaussianbeam profile is irradiated onto the beam shaping element. A Gaussianbeam profile includes a transverse amplitude course that runs throughthe beam center in a Gaussian manner. The FIGS. 33A, 33B, 33C, and 33Dshow respectively the development of the beam profiles 900A, 900B, 900C,and 900D and the respective schematic amplitude courses 902A, 902B,902C, and 902D, the latter directly after the beam shaping element atz=0 mm and at a distance downstream at z=10 mm, z=50 mm as well as inthe focal plane of the successive near field component at z =200 mm. Atransition of 100% is assumed, i.e., one does not generate a strayradiation portion e.g., in terms of non-phase-modulated or scatteredlight.

FIG. 34 shows the amplitude distribution for a step along the beam axisZ beginning at the exit of the beam shaping element at z=0 up to thenear field lens at z=250 mm. The positions of the beam profiles 900A,900B, 900C, and 900D are indicated in FIG. 34 with arrows.

One recognizes that, due to the pure phase mask, a Gaussian beam profile900A and a Gaussian amplitude course 902A are still present directlyafter the beam shaping element similar to the Gaussian beam. A sharplylimited hole is then immediately formed, however, caused by the imposedphase, which yields the divergence. Already at z=10 mm, one recognizes aclear dark spot 904 in the center of the beam profile 900B. The same iscontinuously growing. At the same time, a ring area 906 with higheramplitude is formed.

Ring area 906 is sharply limited towards the inside which can be seen ata step shape in the radial amplitude/intensity distribution. A flank 907of the circumferential step faces towards that beam axis/towards thebeam center. With increasing z values, the opposing sections of flank907 get separated, i.e., the central sharply limited hole grows fast indiameter (D1<D2).

In the radial amplitude/intensity distribution, ring area 906 dropstowards the outside with increasing z values faster and faster. Thisdevelopment is schematically shown in the falling flanks 908A to 908C ofthe amplitude courses 902A to 902C. In the far field, i.e., for examplein the overlapping focal planes of the imposed focusing (far field)action and the near field optics, a sharp ring 908D is formed withinbeam profile 900D, that thereafter diverges (see FIG. 34). Thereby, nowa sharp edge is performed at the outer side, i.e., the step of the innerflank now faces towards the outside.

In FIG. 34, one recognizes the sharp edge in the transition between darkarea 910A, which broadens in Z direction, and border area 910B, whichnarrows in Z direction and is more bright, whereby the grayscale valuesin brighter border area 910B at first have higher values radially insideand then, beginning at the focal plane, have higher values radiallyoutside.

This general behavior of the beam profile and the amplitude coursesenable a test of an optical system with a Gaussian input beam, for whichat first a hole forms with a steep flank facing the inside and therebyresults in a local maximum outside of the beam axis in the far field. Animaging of the beam profile from the inner area as well as in the areaof the focus zone can identify the respective beam profile. The use ofthe optical system is thereby not necessarily limited to Gaussian beams.In addition, it is to note that the figures are a result of calculationsfor the ideal case. For example, if a non-ideal DOE is used, theaddressed non-phase-modulated portion for higher orders or a portion ofa real quasi-Bessel beam (such as for a bi-nary mask) can be on the beamaxis and can fill the “hole” with intensity.

An inverse quasi-Bessel beam can therefore include a step with a steepflank in the amplitude course and accordingly in the intensitydistribution. The same can in particular face to the inside in the areaclose to the beam shaping element, for example, in the area up to halfof the far field, and in particular in the area of a focus length of thefar field optics downstream of the beam shaping element. For a “simple”inverse quasi-Bessel beam without base at the beam axis, theamplitude/intensity increases in the range of the step from almost zeroto the maximum of the phase-modulated beam portion. Thereby, theformation of the step (within the phase-modulated beam portion) is alsogiven for an exemplary incident beam having essentially a constantradial intensity (radial flat top) across the beam shaping element,because the step concerns essentially the beam center.

The beam characteristic described before upstream of the far field focalplane is thereafter radially inverted up to the focus zone. After thatfocus zone, it inverts radially another time such that again a stepshape can be formed at that position—without interaction with a materialto be processed. The beam profile can, for example, be analyzed bytaking the beam at a respective position, be it within the opticalsystem after the beam shaping element or before or after the focus zone.In particular, for setups that allow a blocking of a central disturbingbeam, one can analyze the intensity distribution of the phase-modulatedbeam portion before or after the focus area.

In this context, it is further referred to the German patent applicationentitled “Optical beam shaping element” filed by the same applicant atthe same day that in particular discusses optical systems for beamshaping. The content of that application is herein incorporated in itscompleteness. As is explained therein generally, inter alia inversequasi-Bessel beams can be used for laser material processing.

In connection with FIGS. 35A to 52C, possibilities are presented in thefollowing for influencing the focus zone in longitudinal direction andlateral direction. In particular, the use of a DOE as an example for aphase mask being configured areally enables a simultaneous imposing ofmultiple phase distributions on laser beam 3. For generating an inversequasi-Bessel/Airy beam, a virtual optical image is attributed to atleast one of the phase distributions, wherein the virtual optical imagecan be imaged into an elongated focus area for forming a modification inthe material to be processed by the laser beam. An example of such aphase distribution was provided in connection with FIG. 32. In thepresence of two such phase distributions, that result in at leastpartially overlapping focus zones or focus zones, which at leastinfluence each other, one can shape the geometry of the modification(s)of the material to be processed—generated by a laser pulse or a group oflaser pulses.

In general, such phase distributions can form one or more ringstructures (see e.g., FIG. 37C), a ring segment structure limited to anangular range (see e.g., FIG. 44), and/or one or more local maxima (seee.g., FIG. 49) in a transverse far field intensity distribution (exitintensity distribution).

Several such phase distributions can be imposed in various manners. Anassociation of segments on the phase mask is most obvious (see e.g.,FIGS. 35 A, 35B, and 42). These segments can be separate areal regions,wherein the separate regions can e.g., join radially and/or azimuthallywith respect to each other and can transition into each other, forexample, abrupt or weighted in the border areas. Moreover, the segmentscan be at least partially encapsulated into each other (see e.g., FIG.47). Finally, the phase increase, which is created by an (areal) portionof the phase mask being configured as an area, can be a combination ofphase contributions that are respectively attributed to such a phasedistribution. Besides the DOE configurations described in the following,for example, also respectively combined hollow cone axicons orreflective axicons can reproduce an areal segmentation. One shouldunderstand the following examples for explaining potential conceptsaccordingly, wherein the concepts can also be realized with otherapproaches for phase imposing that were addressed before and are hereindisclosed.

In general, several optical elements can be combined within a DOE, bydetermining e.g., the transmission function of all elements (e.g.,hollow cone axicon(s) and lens(es); adding the individual phasefunctions (exp(−1i (phi1+phi2+. . . )). In addition or alternatively,some type of superposition of individual transmission functions can bedone. For the determination of the phase distributions, it was initiallyreferred to the publication of Leach et al..

FIG. 35A illustrates a phase distribution 930 of a phase mask with twosegments having different phase distributions. One recognizes inparticular a central segment 930A and a ring segment 930B. Ring-shapedrotationally symmetric phase imposing is present in central segment 930Aand in ring segment 930B. The imposing of each of the phasedistributions results in an inverse quasi-Bessel beam, to each of whicha virtual optical image is attributed respectively upstream of the beamshaping element, wherein each of the optical images can e.g., be imagedon another longitudinal area. Accordingly, in particular longitudinal,interference effects can form.

FIG. 35B shows a further example of a phase distribution 932 of a phasemask with two different phases distributions that are arranged in radialsegments (central segment 932A and ring segment 932B). Herein, each ofthe phase distributions is additionally superposed with a sphericalphase in comparison with FIG. 35A, so that the phase mask has a farfield focusing action. Again, the phase distributions are rotationallysymmetric. Both phase distributions from inverse quasi-Bessel beams withfocus zones in comparable areas on the beam axis.

For ring segment structures, FIG. 36 shows an amplitude distribution fora cut along beam axis Z beginning at the exit of the beam shapingelement at z=0 mm up to the near field lens at z=250 mm. One recognizesprincipally two overlapping amplitude distributions with differingdivergence that each correspond in their characteristics to theamplitude distribution shown in FIG. 34. In particular, one recognizes acentral, dark, and in the Z direction broadening area 934A as well as anouter, dark area 936A that extends ring-shaped and widens in Zdirection. Bright and ring-shaped beam areas 934B and 936B borderradially to the outside to dark areas 934A and 936A. In particular,intensity distributions form that have a step-shape in the radialdirection that are explained in particular in connection with FIGS. 37Ato 37C and FIGS. 38A to 38C and can change their characteristics in thearea of the far field focal plane. It is noted that the far field focalplanes for those ring segments do not need to overlap.

FIG. 37A shows a beam profile 940A that forms at z=10 mm. One recognizesthat a central dark point, which corresponds to central dark area 934A,and a dark ring, which corresponds to ring-shaped dark area 936A, haveformed in the originally Gaussian beam profile of the incident laserbeam.

FIG. 38A shows the transverse intensity distribution associated to FIG.37A. One recognizes in particular a pair of opposing central steepflanks 947A. These represent the transition in space from bright area934B to central dark area 934A and face accordingly radially towards theinside. Bright area 934B includes a slowly descending flank 948A in theradially outside located area. Descending flank 948A continues in asimilarly slowly descending flank 948B of bright ring-shaped beam area936B on the other side of dark ring area 936 A.

FIGS. 37B and 38B show a beam profile 940B and a radial intensitydistribution 942B at z=50 mm. One recognizes that the intensity areascondensed further in the radial direction, so that central dark area934A and dark ring area 936A became radially larger. In FIG. 38B, onerecognizes in particular a steep flank 947B of outer bright area 936that faces towards the inside as well as above noted slowly descendingflank 948 that adjoins radially to the outside.

FIGS. 37C and 38C show a beam profile 940C and a radial intensitydistribution 942C in the longitudinal area of the focal planes at aboutz=200 mm, which is associated to segments 932A and 932B. One recognizesnow a pronounced ring structure with an inner ring 949A and an outerring 949B. Each of the rings 949A and 949B creates an inversequasi-Bessel beam profile in the focus zone, wherein again interferencemanifestations can form. In particular, this plane corresponds to anangle spectrum of the beam in the focus zone. The angle spectrum showstwo peaks, so that exactly two angle portions are contained in the beam,one angle portion from the inner and one angle portion from the outerarea of the phase mask. The two angle portions result in two-beaminterference in the focus zone.

FIGS. 39 to 41 show an example for an interference for a phase imposingwith radially arranged segments, wherein the interference is based onthe superposing of two focus zones with a small phase difference in Zdirection. In the underlying phase imposing, each segment isadditionally accorded a homogenization, similar to the embodiment of thelongitudinal quasi-homogenized flat top intensity distribution 561,explained in connection with FIGS. 23 to 26.

In general, the introduction of multiple angle portions can causeinterference, wherein this takes place under maintenance of the beamenergy/fluence, so that a high efficiency at the formation ofmodifications can be maintained.

FIG. 39 shows a common elongated focus zone 977 for the twosegment-specific images of the virtual optical images, in which thecontributions of the two radial segments superpose while interfering. Inpropagation direction, common elongated focus zone 977 includes asequence of intensity maxima 961, which are present over an area ofabout 600 μm in Z direction with almost comparable intensities. Onerecognizes that the density of the interference maxima 961, i.e., thefrequency of the longitudinal modulation, increases in Z direction. Themodulation can become e.g., uneven, if not only two angle portionsinterfere, but multiple angle portions are included, for example, due tothe homogenizing. The latter can in particular influence the period ofthe modulation. For exactly two angle portions, the period of modulationwould be constant.

As the segments are rotationally symmetric, also the intensitydistribution is rotationally symmetric and each interference maximum 961corresponds to a volume area, in which the intensity/fluence can beabove a threshold intensity/fluence. FIG. 40 shows additionally arespective ZR-cut 963 through the intensity distribution that forms inthe focus zone and includes successive interference maxima 961.

For laser material processing with such an intensity distribution, FIG.41 shows modification zones 965 of a modification in processed material9, which is associated with a pulse/a pulse group, wherein themodification zones extend in Z direction and include localized anddisplaced modification areas 965A. Modification areas 965A areattributed to interference maxima 961.

In FIG. 41, the upper four images show the formation of modificationareas 965A in the range of the threshold intensity at pulse groupenergies, which were, for example, also applied for the experimentalstudies shown in FIG. 6. In comparison, one recognizes that due to themodulation by interference the peak intensity is larger than the sum ofthe intensity in the partial areas and accordingly larger than for asimple inverse quasi-Bessel beam with comparable angle. For that reason,in comparison to FIG. 6, the modification threshold in FIG. 41 haspassed already at smaller pulse energy (the latest at the secondEg-value) significantly earlier, although it is not stronger focused.

The lower four images in FIG. 41 show the formation of modifications, asthey are created when irradiating pulse groups with higher and furtherincreasing pulse group energies. One recognizes an extension andconnecting of modification areas 965A, because the modulation depth ofthe intensity in the longitudinal direction extends at higherintensities only in very short areas below the threshold and, therefore,only longitudinal short areas do not result in modification of material9. One recognizes also in FIG. 41 that the end of modification zones 965(the position of the last modification area) is essentially stationaryin Z direction for increasing pulse group energy. A similar behavior isexpected for variations in the beam diameter of the incident laser beamwith respect to the position of the last modification area, because thiscan be a typical feature of an inverse quasi-Bessel beam shape.

FIG. 42 shows a face distribution 970 of a phase mask with azimuthalsegmentation. A pair of “X” segments 970A are opposed to each other—eachin triangular shape with a rectangular triangular peak positionedrespectively in the center (corresponding to the beam axis). The areasof the phase mask positioned between “X” segments 970A form two opposing“Y” segments 970B—also in triangular shape with a rectangular triangularpeak positioned in the center. In general, an incident Gaussian beam isdirected onto the beam shaping element such that the center of the beamshaping element coincides with the beam axis of the incident beam.

In the example of FIG. 42, the transition of the phase distributionsbetween the individual segments takes place abrupt. The phasedistribution of FIG. 42 is obviously not rotationally symmetric, becausebeam portions along the X direction are subject to the phasedistribution of the “X” segments 970A and beam portions along the Ydirection are essentially subject to the phase distribution of the “Y”segments 970B.

FIGS. 43 and 44 show an intensity distribution 971A in X direction and acentral portion 971B of an XY-view on an intensity profile as they canform in the far field focal plane. As the phase mask of FIG. 42 does notinclude a focusing phase portion, the same is used with a separate andtherefore for both segments identical far field optics. In intensitydistribution 971A and in portion 971B, one recognizes a two-part outerring segment 972A of an intensity enhancement being located radiallyoutside. In portion 971B, one further recognizes a two-part inner ringsegment 972B of an intensity enhancement being located radially inside.The latter includes essentially no contribution in X direction (y=0).Accordingly, it is also not viewable as an intensity enhancement inintensity distribution 971A. Each part of ring segments 972 A and 972Bextends over 90°—according to the azimuthal segmentation.

In consequence, the azimuthal segmented phase mask of FIG. 42 results inan asymmetric intensity distribution in the far field. Moreover, alongitudinal interference structure can form due to differing angleportions. The asymmetry in the beam shape originates from that asymmetryin the segments. For identical angle portions in the segments and aphase shift of the segments of PI, e.g., an asymmetric beam shapewithout modulation can form, for which the distance of the inversequasi-Bessel beams created thereby can be in the range of the beamsthemselves. The interference of the respective inverse quasi-Bessel beamshapes can accordingly result in an asymmetry/modulation in thetransverse formation of the intensity distribution.

Exemplarily, FIG. 45 shows a cut 973A in ZX plane through a commonelongated focus zone 973 of an intensity distribution, which originatesfrom an output intensity distribution according to FIG. 44. FIG. 46shows a respective cut in ZY plane 973B of the intensity distribution.One recognizes sequences of essentially linearly arranged intensitymaxima 975. Intensity maxima reach significant intensities in a singlerow in FIG. 45 and in three rows in FIG. 46. In FIG. 46, the maxima ofthe outer rows in Z direction are thereby displaced with respect to theinner row. If one selects now the Y direction as feed direction for thematerial processing, then a single laser pulse (or a group of laserpulses) forms a focus zone/modification zone that is elongated in feeddirection, i.e., is asymmetric. Accordingly, the width of the focuszone/modification zone is reduced in the separating direction, i.e., inthe YZ plane. Accordingly, the result is an arrangement of three“cutting” rows of intensity maxima 975.

In other words, the asymmetry, which was created by the segmentation ofthe phase mask, in combination with inverse quasi-Bessel beam shapes canbe used for the formation of a geometric direction of preference whenseparating. Also in this configuration, the end region of the focuszone/modification zone can be essentially independent from theirradiated energy and the beam diameter of the incident beam.

A further example for an interaction space in material 9 havingasymmetric geometry is explained in connection with FIGS. 47 to 52C.FIG. 47 shows a phase distribution 1043 of a phase mask that is based ona superposition of two phase distributions. Each of the phasedistributions belongs to an inverse quasi-Bessel beam as each can begenerated, for example, with a hollow cone axicon. However, the centersof the phase distributions in X direction are displaced with respect toeach other by Ax. Phase distribution 1043 includes further asuperposition with a centrally arranged spherical phase, i.e., afocusing far field action is integrated in the phase mask being e.g.,configured as a diffractive optical beam shaping element.

FIG. 48 shows an amplitude distribution for a cut along beam axis Z inthe range from z=0 mm to z=250 mm as it can be the result of imposingphase distribution 1048. Similar to FIG. 34, one recognizes a darkcentral area 1010A that widens in Z direction. Due to the only smalllaterally displaced phase distributions, diverse interference structures1034 are formed in the bright intensity area 1010B that adjoins radiallyto outside central area 1010A.

The focusing far field action of phase distribution 1048 forms a ring inthe respective focal plane, which is structured in its intensity. Arespective output intensity profile 1051 is exemplary illustrated inFIG. 49. One recognizes local maxima, the extent of which is the largestin azimuthal direction in the X axis. The azimuthal extent decreaseswith increasing distance from the X axis and along the ring.

FIG. 50 shows a ZX cut through the longitudinal intensity profile withinthe interaction area, as it is the result of the focusing of outputintensity profile 1051. In particular, two elongated focus zones 1007Aand 1007B are formed that are displaced in X direction and that extendin Z direction. Besides the main maximum, respective multiple secondarymaxima are formed. The pulse energy or the pulse group energy can be setsuch that, in particular for nonlinear absorption, only the strongestmaximum or the strongest of maxima of each focus zone results in amodification of the material.

If one scans the laser beam, which was formed in that manner, in Ydirection over a material to be processed, a track of two modificationzones at a distance is formed. Thereby, an intended tension distributionwithin the material can be created, which can e.g., start a separationpreferably within an intermediate area 1050 between the elongatedmodification zones. For example, pressure tensions can build up in themodification zones, which result in the formation of tensile stress inthe intermediate area that then supports the respective separatingprocess. Here, the X direction would be again the separating directionand the Y direction would be the feed direction.

The development of intensities in their respective optical systemdownstream of the diffractive optical beam shaping elementwill—corresponding to an inverse quasi-Bessel beam shape—again have astep structure in the radial intensity distribution. Due to the lateraldisplacement of the beam portions for the two inverse quasi-Besselbeams, interference structures 1034 form, however, which can overlaywith the step structure.

Despite the interference structures 1034, one can recognize in beamprofiles 1040A to 1040C for z=10 mm, z=100 mm, and z=150 mm, which arereproduced in FIGS. 51A to 51C, areas that have higher intensities atthe radial inner side. FIGS. 52A to 52C show respective intensitydistributions 1042A to 1042C that extend radially in X direction. Inparticular, in FIGS. 52B and 52C, one that recognizes the formation of asteep flank 1047 that surrounds an inner area of lower intensity.Herein, the intensity radially decays to the outside with a slowlydecreasing flank 1047. However, the formation of the flanks is stronglydependent on the direction due to the interference, as it is shown, forexample, in FIGS. 51A to 51C.

The above explained examples are based on values of two phasedistributions provided on the phase mask. However, more than two phasedistributions can be provided. For example, more than two phasedistributions can be provided in radial and azimuthal segments, or canbe included in combinations of phase steps.

Further embodiments and/or further developments and aspects aresummarized in the following:

In general, the herein disclosed focusing elements such as the far fieldoptics and the near field optics can be configured as, for example,lens, mirror, DOE, or a combination thereof.

Moreover, additional optical elements can be inserted into opticalsystems such as the herein disclosed embodiments. Inter aliaintermediate images can be inserted in the imaging system, to be able torealize, for example, a filter function as well as a scan movement inthe area of the image-side focal plane. Thereby, e.g., the image-sidefocal plane (e.g., image plane 340 in FIG. 20) can itself be imaged byan additional optical system. Alternatively or additionally, suchoptical intermediate systems can allow, for example, realizing anenlarged working distance and/or a magnification of the working fieldfor scanner application.

It is explicitly stated that all features disclosed in the descriptionand/or the claims are intended to be disclosed separately andindependently from each other for the purpose of original disclosure aswell as for the purpose of restricting the claimed inventionindependently of the composition of the features in the embodimentsand/or the claims. It is explicitly stated that all value ranges orindications of groups of entities disclose every possible intermediatevalue or intermediate entity for the purpose of original disclosure aswell as for the purpose of restricting the claimed invention, inparticular as limits of value ranges.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A diffractive optical beam shaping element forimposing a phase distribution on a laser beam that is intended for laserprocessing of a material, which is essentially transparent for the laserbeam, comprising: a phase mask that is configured for imposing aplurality of beam shaping phase distributions on the laser beam incidenton to the phase mask; wherein a virtual optical image is attributed toat least one of the plurality of beam shaping phase distributions, thevirtual image being imagable into an elongated focus zone for creating amodification in the material to be processed.
 2. The diffractive opticalbeam shaping element of claim 1, wherein at least one of the pluralityof beam shaping phase distributions is configured such that a ringstructure, a ring segment structure limited to an azimuthal angularrange, and/or a local maximum is formed in a transverse output intensitydistribution.
 3. The diffractive optical beam shaping element of claim1, wherein at least one of the plurality of beam shaping phasedistributions is configured such that an incident laser beam having aGaussian intensity distribution is transferred into at least onedivergent beam area attributed to the virtual optical image, thedivergent beam area comprising downstream of the diffractive opticalbeam shaping element a transverse intensity distribution, whichdecreases from the inside to the outside and is present in particularbefore a far field focal length (fF) attributed to a focusing action ofthe phase mask, and/or at least one of the plurality of beam shapingphase distributions is configured such that an incident laser beam istransferred into at least one divergent beam area attributed to thevirtual optical image, the divergent beam area comprising downstream ofthe diffractive optical beam shaping element a transverse intensitydistribution that comprises a section of a step-shaped intensityincrease, which comprises a steep flank facing radially to the inside,and that is present in particular before a far field focal length (fF)attributed to a focusing action of the phase mask.
 4. The diffractiveoptical beam shaping element of claim 1, wherein a virtual optical imageis respectively attributed to several phase distributions of theplurality of beam shaping phase distributions and the optical images areconfigured for being imaged in respective focus zones in order to shapein combination the modification in the material to be processed, and/orthe phase increase generated by a section of the phase mask, which isconfigured areally, is a combination of phase contributions, which arerespectively attributed to the plurality of beam shaping phasedistributions.
 5. The diffractive optical beam shaping element of claim1, wherein the phase mask, which is configured areally, comprises aplurality of segments that are respectively configured for imposing asegment-specific phase distribution of the plurality of beam shapingphase distributions on the laser beam, at least two segment specificphase distributions are associated respectively with a segment-specificvirtual optical image that is imagable in a segment-specific focus zone,and the respective segment-specific focus zones are arranged withrespect to each other such that they contribute together to theformation of a modification zone.
 6. The diffractive optical beamshaping element of claim 1, wherein the plurality of segments comprisesat least two segments that are composed of spatial structures that areat least partly encapsulated into each other; and/or segments of theplurality of segments join radially and/or azimuthal; and wherein inparticular a weighted transition between the respective neighboringphase distributions is set in the transition area of neighboringsegments of the plurality of segments.
 7. The diffractive optical beamshaping element of claim 1, wherein the, in particular segment-specific,focus zones are superposed with respect to each other and/or spatiallycomplement each other; and/or at least two, in particularsegment-specific, images of the virtual optical images are superposedwhile interfering; and/or at least two, in particular segment-specific,images of the virtual optical images form a common elongated focus zone.8. The diffractive optical beam shaping element of claim 1, wherein the,in particular segment-specific, phase distributions of the plurality ofbeam shaping phase distributions are configured such that the images ofthe respective optical images result in an asymmetric transverseintensity distribution in the material to be processed, in particularbased on an asymmetry of a focus zone itself and/or on the asymmetricarrangement of multiple focus zones with respect to each other; and theasymmetric transverse intensity distribution comprises in particular afeed intensity distribution in a feed direction and a separatingintensity distribution transverse to the feed direction and wherein inparticular the feed intensity distribution extends transversely furtherthan the separating intensity distribution and/or the separatingintensity distribution comprises two intensity maxima for separating thematerial in between the intensity maxima, and the intensity maxima inparticular relate to two neighboring, in particular parallel withrespect to each other, extending focus zones.
 9. The diffractive opticalbeam shaping element of claim 1, further comprising: a separate farfield optics and/or a far field optics phase imposing integrated intothe phase mask, wherein the far field optics and/or the far field opticsphase imposing is configured to perform a focusing action on the laserbeam, in order to form at least the one virtual optical image in arespective focal plane, and wherein in particular the focusing actionfor different phase distributions of the plurality of beam shaping phasedistributions is identical or different, and is in particular differentfor different segments.
 10. An optical system for beam shaping of alaser beam for processing an in particular transparent material bymodifying the material in a focus zone that is elongated in a commonpropagation direction, the optical system comprising a diffractiveoptical beam shaping element of claim 1; and a near field optics, whichis arranged downstream of the diffractive optical beam shaping elementat a beam shaping distance (Dp) and configured to focus the laser beaminto the focus zone, wherein at least one imposed phase distribution ofthe plurality of beam shaping phase distributions is such that a virtualoptical image of an elongated focus zone is attributed to the laserbeam, the optical image being located before the diffractive opticalbeam shaping element, and the beam shaping distance (Dp) corresponds toa propagation length of the laser beam within which the plurality ofbeam shaping phase distributions transform the transverse inputintensity profile into a transverse output intensity profile in theregion of the near field optics, and the transverse output intensityprofile has in particular, in comparison with the input intensityprofile, at least one local maximum lying outside of a beam axis. 11.The optical system of claim 10, wherein the diffractive optical beamshaping element is arranged to be rotatable around a beam axis of theincident laser beam, in particular to set an asymmetric transverseintensity distribution in the workpiece in its orientation with respectto a preferred direction, in particular a feed direction; and/or whereinan imaging system attributes to the beam shaping element an image planedownstream of a longitudinal center of the image of the virtual opticalimage, and a transverse beam profile of the laser beam is present at thebeam shaping element in the image plane, and wherein there is inparticular in the region of the image plane a change, which changes fastin longitudinal direction, from a lateral beam profile, which is givenin the focus zone, to a lateral beam profile having a dark center, thelatter in particular for an essentially lateral Gaussian beam profile ofthe laser beam and in particular with respect to beam portions of theincident laser beam, which generate a divergent beam area that isattributed to the virtual optical image; and/or the optical system isconfigured such that essentially only a central area of the incidentlaser beam contributes to a downstream end of the focus zone attributedto the virtual image, so that a change of the beam diameter of theincident laser beam does not result in an essentially longitudinaldisplacement of the downstream end of the focus zone.
 12. A laserprocessing machine for processing a material, which is in particular toa large extent transparent for the laser beam, with a laser beam bymodifying the material in a focus zone, which is elongated in a commonpropagation direction of the laser beam, comprising a laser beam source;an optical system according to claim 10 comprising a diffractive opticalbeam shaping element; and a control unit for setting operationparameters in particular for orienting a transverse asymmetric intensitydistribution with respect to a preferred direction, in particular a feeddirection, wherein the control unit is configured in particular forsetting a rotation angle of the diffractive optical beam shaping elementwith respect to the beam axis of the incident laser beam.
 13. A methodfor material processing a material, which is in particular to a largeextent transparent for the laser beam, by modifying the material with alaser beam, the method comprising: imposing a plurality of beam shapingphase distributions onto a transverse input intensity profile of thelaser beam, wherein at least one of plurality of the imposed phasedistributions is such that a virtual optical image of an elongated focuszone is attributed to the laser beam; propagating the laser beam over abeam shaping distance (Dp), after which the plurality of imposed beamshaping phase distributions has transferred the transverse inputintensity profile into a transverse output intensity profile, so thatthe transverse output intensity profile, in comparison to the inputintensity profile, comprises in particular at least one local maximumlocated outside of the beam axis; and focusing the laser beam into thefocus zone for forming a near field, which it is based on the outputintensity profile, while superposing, adding, and/or interfering theelongated focus zone attributed to the virtual optical image with atleast one further focus zone, which is based on at least one furtherphase distribution of the plurality of beam shaping phase distributions.14. The method of claim 13, wherein, in particular for a nonrotationally symmetric shaped phase distribution on the phase mask,which is formed in particular by a non rotationally symmetricarrangement of the plurality of phase distributions on the phase mask,an asymmetric transverse intensity distribution in that material isformed by focusing the laser beam; and/or the method further comprisesthe step of aligning an asymmetric transverse intensity distribution, inparticular by rotating the beam shaping element, with respect to apreferred direction, in particular a feed direction.
 15. A use of acommon elongated focus zone for laser material processing of a material,which is in particular to a large extent transparent for the laser beam,by modifying the material, wherein the common elongated focus zone iscreated by spatial adding and/or superposing of several inversequasi-Bessel laser beam profiles and/or inverse quasi-Airy laser beamprofiles.
 16. The use of the common elongated focus zone of claim 15,wherein the spatial adding and/or superposing results in interferencesand/or intensity modulations in the common elongated focus zone and/orin an asymmetry in the common elongated focus zone.